You need to search for !nixos@lemmy.ml or https://lemmy.ml/c/nixos then it will either show up in the search result or you can use https://programming.dev/c/nixos@lemmy.ml to subscribe from there.
So normally the answer to this question is that you need to search using the ! syntax, then potentially wait a little bit (if you're the first user from your home instance to ever search that community) and then the "no results" will update and it'll work.
HOWEVER this seems to be a new problem with lemmy.ml specifically. This morning I've tried and failed to search for multiple lemmy.ml communities from both lemm.ee and lemmy.world. Both repeatedly return no results, and they're also not silently working behind the scenes as visiting what should be the local URL still 404s.
So there's a new problem, and so far I've only seen it with communities hosted on lemmy.ml. Could this be an issue with v0.18? Anyone else care to test this behaviour and report back?
I think Lemmy itself should have a built-in search for all instances. This is the biggest source of confusion when you are first trying to find stuff to subscribe to.
If it has never pulled out from that instance, you have to search for the full name the first time. In this case it is !nixos@lemmy.ml
