IRC, XMPP, Matrix, Rocket.Chat.
Maybe you'd want to skip IRC if message retention is an important detail. Although, I'd argue the local-logging method is more reliable and faster to search through than what most modern chats give you.
If you're hosting on your own hardware locally, XMPP will be less straining on resources than Matrix, at least the last I heard.
I haven't actually used Rocket.Chat, but I threw it on the list because I think it's aiming more for the Slack crowd than these other more general options.
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IRC, XMPP, Matrix, Rocket.Chat. Maybe you'd want to skip IRC if message retention is an important detail. Although, I'd argue the local-logging method is more reliable and faster to search through than what most modern chats give you. If you're hosting on your own hardware locally, XMPP will be less straining on resources than Matrix, at least the last I heard. I haven't actually used Rocket.Chat, but I threw it on the list because I think it's aiming more for the Slack crowd than these other more general options.