With regards to hashtags I think the utility is mostly in searching among similar things within a community. Suppose there's a community that serves a purpose like r/askhistorians, stackexchange, or like what I'm trying to do over at !opencourselectures@slrpnk.net. In each of those cases, it is enormously useful to be able to search the community by subtopic. Obviously, this could be solved in other ways, but hashtags are probably the simplest to understand and implement.

Very excited to see the outcome of your Ibis project, but I think Lemmy native wikis would see significantly more activity. The easiest implementation I can imagine is a slightly altered frontend for communities marked wikis that also handle some well known syntax (like [[link]] popularized by pkm systems) for internal links. That is links to posts by the same name within a community.

Currently, I'm working on a lemmy bot that handles exactly this internal linking, and hashtag functionality, then builds a static site with support for github pages (so the end result is both a linked community and a seperate site), but I'd much rather have this functionality built into lemmy. To be frank, I'd much rather be trying to build this functionality into lemmy and if I wasn't nearly certain it'd get shot down as out-of-scope, I'd probably be doing that.

I know you're not particularly fond of growth based arguments for new features, but I sincerely believe that the thing that made reddit great in those early days was the tendency for communities to compile resources (particularly for niche and hobby communities). This gave the communities a certain depth that is nearly impossible with posts alone. If that were a first-class feature of Lemmy, I think you'd very quickly see Lemmy fill the niche that federated wiki projects and supplementary wiki services have so far failed to.

hashtags have better use in platforms like mastadon

Says who? Because that's how the old platforms used it? I think we should really be moving past the direct influence of these corporate platforms and on the fediverse that probably means something of a well understood common language (like @user@instance or !group@instance). Hashtags were only ever a thing on twitter because the users just started using them and full text search was sufficient to handle it (this is more or less the position we're in on lemmy). Direct support didn't come till much later.