I'm missing pretty much all the "League of Legends Mains" communities, especially ADCMains and SamiraMains. They were one of my absolute go-to subreddits and the only reason I haven't fully deleted my account yet.

Other then that it's mostly niche things, especially very niche wholesome fem-oriented NSFW subs

Scribus. I still use the open source DTP at work even if I have InDesign installed.

Edit: they have been added, I'm thrilled r/lockpicking or room porn if they haven't been added.

DepthHub & RBI were good at times.

Although the obvious answer for me is the already niche subs on Reddit for software/TV shows which have just a few hundred people there. There'd be 10 people at absolute max on here and the community would not last.

PublicFreakout. I love cringing at the worst people in society

destinythegame

I miss /r/opensuse and an active Java community. There are a few Java communities on several servers, but they are all basically dead.