Just wondering why the ! symbol is used to mark a community here on Lemmy, but not #, + or something else? Was there any discussion and mentioned reasons? Thanks in advance.
That used to be the convention for federated groups dating back to GNU-Social / StatusNet and has also been used for groups in Fediverse software such as Friendica and Hubzilla.
It is also a lot different from # tags, so it would not be a good idea to use those.
However it seems like for compatibility reasons some Fediverse software is switching to @ based direct messages for group posting (The group account then automatically reposts on behalf of the user). But I am not sure how that will translate to a groups first software like Lemmy.
Ah, I can remember that on Friendica. I see the point, the # symbol is for tags, so groups require something else. So then the Matrix approach with # for group chats and + for spaces therefore lacks a symbol for tags should they require one. I think group reposting bots are a temporary solution anyway.