Originally the Forum was on Nifflas's website: it was moved to the current host before the release of Saira to avoid service problems. Since then, the Forum underwent some small modifications in structure and rules. The old Forum remained accessible (but no new posts were allowed) for a while, then it was closed (access allowed only for administration purposes).
In the old Forum, you could attach KS levels directly to level topics, provided the level was smaller than 500KB; larger levels were uploaded to other sites, mostly file hosting sites. The Knyttlevels archive was created before the move to the current Forum, I believe, but many levels remained as topic attachments, and since some authors abandoned the Forum before the move, their levels are now either inaccessible or the file hosting site they used for them removed them. Therefore, a lot of levels were 'lost' this way.
While a number of levels were rehosted on Knyttlevels, there are probably many which are only found in some users' personal collection (myself included); we've agreed that there's nothing wrong with reuploading lost levels to Knyttlevels.
LPChip originally planned to remake the level archive to include several nice features, but now Looki is working on it. There are currently no deadlines or anything for the project. If you're asking about what'll become of the currently archived levels once the new archive is made, you'll have to ask LPChip or Looki.