To add to these, albeit not entirely the same, there’s Zulip. I’ve not used it personally, but I love the thinking behind the design, where you have the classic channel style but then also threading so that you can better organize & follow conversations.
I don’t use Discord, but Revolt looks similar
https://revolt.chat/
Others:
https://matrix.org/
https://mattermost.com/
https://github.com/spacebarchat/spacebarchat
To add to these, albeit not entirely the same, there’s Zulip. I’ve not used it personally, but I love the thinking behind the design, where you have the classic channel style but then also threading so that you can better organize & follow conversations.