I don't see any difference between hashtags and community IDs. Their meaning is fluid and can be taken over by the crowd at any point. Unless you filter by a moderating relay, which then both IDs and hashtags also apply and the definition is whatever the relay, not the users, want.
They can all be spammed in the same way. The stronger they are, the more spam they are going to get.
there's no difference in terms of spam prevention, its really just to have an anchor for the creator to define the name, banner, and description. if you don't care about those things you can just use a hashtag (#physics) or namespace (!sci.physics.quantum) anchor.
I've always wanted explicit to/cc on notes, so I would throw that in there as well for relay filtering and better hinting:
https://github.com/damus-io/notedeck/issues/788#issuecomment-2734024701
this is a combining my original mailing list idea with communities i guess
also with a user anchor they can also update the description with addressable events, as well as create relay lists for finding the canonical location, etc.
the forking and moderation slicing is undefined and can be grown onto the spec later ?
