Mainly these two:

https://github.com/processone/fluux-messenger https://github.com/prose-im/prose-app-web

both also have non-web apps and generally work with self-hosted xmpp servers. But due to various factors I would not recommend them right now. But they are worth a try.

And https://movim.eu/ is also evolving into more of an Discord alternative, but not as suitable for a “company chat”.