I never worry about presence in any form of chat. I don't care if people are there when I message them; it's up to them to pick up my messages at their convenience.
On a forum, every conversation is within a thread for example.
Chat is unthreaded by default, even if Slack allows threads. It also allows users to create channels and manage who's on those groups, while the forum equivalent is fixed up-front and only opt-in.
So really chat is a forum that's a bit more free-form.