There were a bunch of apps back in the day that could hook into MSN Messenger to provide a whole bunch of functionality like this. It could tell you when people closed their chat windows.

Yeah we're trying to tackle the usability stuff in Pidgin 3. Unfortunately due to API guarantees we just can't make the changes people want in purple2/pidgin2, so it is what it is. We are nearing an alpha soon (tm) (no date set) but they goal is to have basic pidgin 2 functionality with the ability to add all the new features.

I have a public blog post on my patreon that goes into some more detail if you're interested. https://www.patreon.com/posts/what-were-with-3-78291546

Can't wait to see what Pidgin 3 is going to be like.

How bad are the API changes? Will people be able to use existing libpurple implementations?

It's essentially a brand new program/library now.

A lot of thirdparty clients did that, and it's a natural consequence of the protocol relying on creating sessions (ephemeral chat rooms is a good description of what they're like), which its participant(s) are notified of being added to prior to the first message being sent; but the official client just keeps track of the session creations and doesn't actually indicate any activity in a session until a message is sent.

Kopete? I also remember some Linux MSN clients at the time actually told you when some contact closed the conversation window. That was really cool.