I've had several people contact me with alternatives, and admittingly they all have have issues or lacking features that make people want to switch. This list is from
@Koi Fish
- Revolt.chat
- Matrix Element
- Slack
- Rocket.Chat, Mattermost, Zulip and other Slack-inspired programs
- Telegram
- Rumble
- Raidcall
- Overtone
One more I haven't seen mentioned so far I'd like to add is
Tox (uTox / qTox that I know)
I haven't given that one a proper run since I haven't seen as many using it,
but it's got voice call, video call, file transfer, Groups, and seems pretty darn secure.