FYI it's considered bad form to post the cheat code on the forum (which is largely why it became famous in the ways Ix included in the chart).
I'll give it a shot...
"1.0.4. Installer version" I'm guessing it's just one of the earliest builds since it's currently on 1.2.1
I actually have no idea. Good start, Ix."Ate the Mushroom"
Reference to objectively the greatest KS level of all time, UncleSporky's 'Don't Eat the Mushroom'."KS+" A community made launcher that has some added options and the abilities to create maps not possible in the original.
Egomassive's mod - originally created just to add functions to KS that he wanted for his (never finished) level 'The Tree of Knowledge'; when he decided to also create a level editor allowing designers to make use of his features, and to take user suggestions for additional functions, he ended up making the most popular KS mod."KS Ex" Similar to KS+ with included Level Editor and other add-ons
Another KS mod, less popular as it required designers to have actual coding ability to use."Juni in Knytt Underground" - Not sure. Guessing you can either play as her or she shows up as a cameo in Nifflas' other game "Knytt Underground"
A secret in Knytt Underground allows you to play a bonus level as Juni."ni2.se before deletion" - Old website? I heard a rumor that he wasn't supportive of the fan levels or something like that.
A few years ago Nifflas suddenly decided to delete his website and withdraw from the internet, overwhelmed by all the trollishness. When he changed his mind on this a few days later, he realised he no longer had a backup of his site, and he has never made a replacement.Gradient0.png - No clue.
In the KS editor, default gradient 0 is the purple void screen background. If you've ever been playing a KS level and stumbled into an area the level designer did not intend you to be able to access, you've probably seen this.Picking up the key - references the level "Do Not Pick Up the Key" by Talps
I'm not sure on this one; considering all the warnings in the level, I don't think anyone would have been dumb enough to actually do this... 
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"KS as illegal music distribution tool"
People including copywrited music in their levels, allowing for unlawful distribution (I would never do this of course *ahem*)."Double tap down for holo" - I didn't know you could do this as opposed to just pressing "E"
I think this is one that a lot of people discover by accident; I don't think it was mentioned in the tutorial."It waits" - creepy level created by Colossal
"Knytt Stories DS" - freeware DS game, never played it
"ORTOFAKTOR" - tileset mainly used in Knytt Underground
"Proto Jon" - does Let's Play videos of KS
"Unreleased music" - No clue.
There were a few tracks composed for KS (and/or its discontinued sequel) that were not included in the game. They have been posted on the forums so you could probably track them down."Lost forum levels" - May be talking about the lost levels that the community tried to find the files for a while back
A loooong time ago the Nifflas forum crashed catastrophically and all its content was lost (I think; this was before my time). A lot of levels were orphaned as a result; the links to them were lost. Players have dedicated a lot of time to hunting for the lost levels (often known only by title and author) on other websites or in other players' collections. Like the lost episodes of sixties 'Doctor Who', there are many that are probably lost forever."Cheat code in Explore Challenge" - Possibly [REDACTED] which allows you to be invincible when holding space, and can [REDACTED]
Most players first discovered the KS cheat existed at all by finding a secret area in 'The Explore Challenge', as this custom level was (at one point at least) hosted on Nifflas' old website alongside the download for the game itself."Ball mode" - No idea.
KS Ex (I think) gives the player the ability to transform into the ball from 'In a Deep Forest', as in Knytt Underground.Code symbols interpretation - No idea.
Reference to the symbol buttons that are used to unlock doors in some levels, famously used in 'An Underwater Adventure'. Since most people find abstract shapes difficult to memorise quickly, most players interpret them into words that are easier to hold in their short-term memory. They then argue with one another as to the correct word to use for each symbol (which is ridiculous, since they are obviously ball, kick, parallel, plus, tunnel, right, smile, corner, and left)KS Advanced - No idea. Possible port to Gameboy Advanced?
Another KS mod, made by Purple Pineapple I believe."Hidden Star Inn" - No idea. Maybe a forum section only available to those who were gifted hidden stars?
There is definitely not a hidden board in this forum..."Teenhmifnoeafgil" - Trippy level with an awesome boss fight. Made by KG
"Knytt Nano" - originally going to be a sequel to Knytt and was included in the Nano Pack.
"When we're dead" - Old custom level
"Sinrevah stalks Juni in different levels" - No idea.
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"Red flower uncensored" - Reference to the red flower in Saving Thalanill? Hopefully not rule34...
Been ages since I played Thalanill... (sorry, Ix) Though I'm guessing this is a joke that arose from the level being streamed by speed-runners, rather than something in the level itself."Gradient info bar" - No idea.
I'm guessing this is a reference to the fact that if you create a custom tileset, there is a little bar at the bottom where you can credit yourself and any other creators without using up part of the set, and give or withdraw permission for other designers to use your set. Gradients do not have this, so they will generally be used and reused uncredited."The mushroom in Thalanill" - It's been a while since I've played this level so I'm not sure if I met the mushroom or not.
C.f. red flower. Sorry, Ix."When you get the umbrella, rebound" - No idea.
Pretty sure this was a semi-famous bit of Engrish in a level I've forgotten..."KS Randomizer" - tool made by Purple Pineapple that randomizes enemies, objects, etc. in levels for replayabiltiy.
...replayability and chaos."Thalanill bad ending strat" - No idea.
I've two thoughts on this one. Firstly, when 'Saving Thalanill' was first released it was impossible to get the good ending, because both paths linked to the bad ending (there was another glitch in Thalanill upon release, but WE DO NOT SPEAK OF THAT ONE, DO WE IX? 
).
'Thalanill' has been speed-run (speed-ran? speed-runned?) on Twitch; taking advantage of glitches in the game allows speed-runners to skip a significant part of the level and save a big ol' chunk of time; I'm pretty sure this locks them into the bad ending, though based on this, maybe it's the good ending."The K in 'Knyght' is hard"
If Ix can fill this with references to his level, then I can do the same dosh garnit! After I released 'Dragon Myth', several players felt the need to tell me I had misspelled 'knight', overlooking the terribly subtle portmanteau of 'knight' and 'knytt' (knights who are knytts)."KS Level composer" - Tool made by xoft to edit levels
"Level archive animated icons" - No idea
There have been numerous attempts to update the KS level archive with more advanced filters, icons, a rating system... None have ever come to light, despite several players over the years taking the project upon themselves."Hold S" - Jump higher, is that it?
Not sure if Ix was referencing advice in a specific level, or to general advice on how to overcome the game's most basic obstacles."The Empty planet" - Map made by plural
Plural's levels are deep and gorgeous and we all want to play them and own them and get lost in them and hug them and love them. 'The Empty Planet' has never been released, despite Plural stating for many years that it is sitting on the brink of completion."Hologram activated shift" - No idea.
There is a mechanic in the game so that shifts (teleports between screens or parts of a screen) can be programmed to not work when the player has the hologram activated. Despite being part of the default game and very prominently placed in the editor, this is hardly ever used by designers. There is no shift that works in the opposite way, only activating if the hologram is active."A walk at night" - Level made by Quincent Cartographer
An old, tiny, and rather obscure level that will not be forgotten by anyone who plays it. Quincent made some tilesets for other levels but never released the tilesets or the levels - heart-breaking, considering how beautiful his art was."Rapid climbing TAS trick" - A way to climb faster in the Gliperal's TASer Handbook that is way too complicated to explain
"Level archive update" - No idea.
There have been numerous attempts to update the KS level archive with more advanced filters, icons, a rating system... None have ever come to light, despite several players over the years taking the project upon themselves.=======================================
"'Colour' misspelled in forum tags"
Yeah, I'm getting a bit annoyed by this as I write this post. "Toasts saying 'Rawr' and tileset 73 roses"
My own Easter eggs, which no one ever noticed... I put the first toast in 'Eurydice' before Ego released 'A Knytt in Time', so my toasts are not a reference to his, even though everyone thinks they are."Title theme Bardo symbolism" - No idea.
I think this may be a reference to the fairly zen music and the swirly rings of dots on the KS title page.Lost PNG map generator" - No idea.
"Original Forums crash" - No idea.
As above, the original KS forum crashed suddenly, taking with it most/all of its contents, including links to many old levels."Jordan Underneaths video" - Game reviewer?
A Youtuber who made a history-of-Knytt video."Advanced features demo" - No idea.
Rings a bell but I can't remember for sure. I think this is a reference to a level intended to demonstrate the features of a mod."Cheat code in other levels" - Using [REDACTED]
Other designers have occasionally given away the KS cheat in their own levels, though most players will often have learnt the cheat from a less obscure source before finding it in such a way."All toast facts known" - Reference to a very annoying and knowledgeable piece of bread via Egomassive
Egomassive began hiding toast enemies in secret areas of his levels that told facts to the players. These facts were numbered and presumably if you hunted through his levels you could find them all in numerical order. However, as other designers have referenced this Easter egg in their own levels, finding all toast facts ever written would be more difficult than just playing Ego's levels."Every level is canon" - Treating community levels as canon?
The intro to 'The Machine', the first ever KS level, and lore in 'Knytt Underground' imply that the many stories of Juni form a canon of myths that the fey tell of her. There are far too many stories (most of them introducing Juni as a simple non-adventurous girl), encompassing far too many contradictory genres and time periods, for them to all be true about a single person. Also I don't think I'd be wrong in saying that the majority of KS levels probably couldn't be adapted or interpreted as folk tales, however you look at them (c.f. abstract and slightly pretentious)."LPChip's 100th star celebration" - No idea. Do I want to know?
About five years ago, some of us set out to make a level to commemorate LPChip becoming the second person to receive 100 merit stars on the forum. The level was about 95% completed before becoming stuck in a near indefinite hiatus - but thanks to the extra time lockdown gave me, the level will finally be released next month!"The K is hard"
Where was this, Ix? The 'k' in 'Knytt' is not a silent letter - the word is pronounced kuh-nit. Most people don't know this."Juni in SMBX" - Juni as a playable character in Super Mario Bros X?