I fully concur. 'Other Projects' should be reserved for projects not related to Lix, NeoLemmix or SuperLemmini.
I find the way that level content is organized is a mess right now anyway. For Lix levels, it's just threads in the Lix subforum. For Lemmix/DOS, there's a dedicated Level Designing subforum. For NeoLemmix, somehow all the levels go into the Other Projects subforum which I find very counterintuitive. Either way it's very inconsistent.
Either, the Level Designing subforum should have a subsubforum for each engine, or each engine should have its own level design subsubforum. I think the advantage of the former is that it allows very consistent treatment of everything including Lemmix, while Lemmix/DOS would still be a special case in the latter version. The main forum could then serve discussion of level design guidelines and levels for any engines that are not Lemmix/Dos, NeoLemmix or Lix, while anything engine-specific goes into the respective subsubforums. Considering how large of an aspect level designing is for the community, it could also be a subsection (on the main page) with subforums rather than a subforum with subsubforums.
The Projects subforums then serve development and engine discussion. I'd consider custom style creation part of development and thus leave it in that subforum. If there's a surge in custom style creation for an engine, it could get a dedicated subsubforum.
With regards to deleting old topics, I'm generally very much against deleting content. If the in-development content swamps the completed level packs, then I'd advocate either having a regularly updated sticky listing completed content with brief descriptions, or a dedicated subsubforum, or closing old topics (I'm generally against closing topics but in this case it could be warranted because there's a newer topic almost any discussion it more relevant for).
On a different note, I found Technical Stuff (from the previous version of the forum) a much more descriptive name than Lemmings Lab (what it is currently).