AIM handled presence quite well- custom "away" messages, easily editable "profiles" which were like a .plan file to include a note about where you were logged in, idle timers, icon to tell if an IM was forwarded to a cell phone. Finally while it lacked anything like Slack has to store your messages centrally and persistently, AIM would notify a user when you logged on to more than one location and allow remote logoff of your other sessions to ensure subsequent messages only went to your current session.

I'm glad you've been blessed with whatever Apple Magic lets iMessage work across devices for you but between my iPad iPhone and MacBook all I can say is I follow up important messages with a phone call because it's one of the more unreliable and indecipherable systems I've never been able to figure out.