Hasn't Mattermost already done one bait and switch?

I don't use it but IIRC there was an uproar a couple of months ago about them just switching up message history and deleting conversations.

Zulip is definitely not an alternative to discord but I'll check out the others.

I don't see why not. I like Zulip. Its is self-hostable and OSS and they're the only ones who have "public channels" that allow your channels to be indexable by search engines on the internet if you want. One of the biggest complaints I hear on HN is how Discord killed the openness of the internet that forum-based communities supported

I wish more communities used Zulip.

There are different types of communities on discord. Zulip will be a good idea for some of them.

Fair enough

For mobile, you can use FluffyChat, it's another Matrix app

There's Element for mobile too. More complete list of clients here

https://matrix.org/ecosystem/clients/