I’ve looked through the history of threads in this help community, but didn’t see this Q come up before.

I also notice someone saying on another instance that as a mod, it’s indeed possible to pin by clicking the three dots under the thread / post. I even see an open github issue discussing cross-instance problems with pinning posts.

Still, here on this instance, it doesn’t currently seem possible to pin in either of my two communities. If it were, I’d want to pin this welcoming post, for example.

EDIT: Thanks to @Terevos@lemm.ee for the solution-- it’s as simple as going in to a thread, clicking the “three dots” in options, then clicking again “community” with the little pin symbol.