Mac

This is true for most providers but not the big big ones. Ask me how I know ;)

You would need to host your own or join a matrix server. The benefit of Matrix is you can tie in puppets to things like iMessage, Discord, FB Messenger if you host it yourself. Another option is Beeper (Matrix + all the puppets for other messaging apps) but you still have to sign up and get an invite from them to join for now.