You forgot XMPP which we used about ten years ago for team chat, and it worked well and still does. The idea being, like with SMS and email and phones and IRC,

You forgot XMPP which we used about ten years ago for team chat, and it worked well and still does. The idea being, like with SMS and email and phones and IRC, that multiple client and server implementations can inter-operate. Especially when chat software has been in use for well over 30 years now.