From their mastodon:
so my goal with sup. is to make a federated snapchat/whatsapp that is compatible with most platforms. This was originally going to be a Pixelfed specific Messenger-like app, but then it hit me - this is a perfect opportunity to create a sub project that could be eventually handed over to a trusted org.
Seems like a fine ambition but Pixelfed is still in beta and buggy as hell. I’d like to see that improve (so that more people will want to use it) first.