Friendica.

but it is not cleanly threaded like lemmy or piefed. a developer said they will improve this a bit in the next version , but certainly not to the level of threadiverse.

Anyway they have all kinds of technical fancy stuff regarding groups, hashtags, saved searches and whatever.

Thank you for the hint on the article and your points regarding my question! Regarding Matrix : yeah, also other guys here pointed me at that. Good to know better now ; before I thought it was part of fediverse. now I know that fediverse is basically defined by some level of communication via ActivityPub.