- every time you see anything relating to a user in the mastodon api, it returns the entire Account entity of said user
- the Account entity contains a last_status_at property
therefore: it is possible to create a psuedo-online-indicator that checks if the last status was within the last few seconds/minutes
additionally, a backend that wishes to have this as a "feature" can keep track of any activity being received from the given actor (likes, reacts, edits, undoing anything) without placing extra load on the protocol with a real-time presence system (which AP is NOT made for in the slightest)