oh, maybe it's cuz I'm on a less populated server for lemmy. I'm on lemmy.studio and it's been good, but it's actually really hard to know about any non-lemmy communities. I believe you can only search up "Redditmigration" on lemmy.world since someone already did the initial work to subscribe to it. On lemmy.studio, most of even the lemmy communities aren't on there yet, but at least the lemmy browser will let you see what's out there, so you can know to do the work to be the first to subscribe to it on that less populated server. However, there would be no way to know about the kbin communities if all I'm looking at is that lemmy browser
I'm guessing kbin would have the same issue( but im on a popular server there, kbin.social, so I can't confirm. Everything here, like you has been searchable for me probably cuz it's big so people have already subscribed)