I would like to know which parts have an online mode, but not to play with friends but modes with random. I once heard of a mode that works so that an assassin chases you and you have to blow one. Nobody knows who is following him. So modes or other fun ones
My second question would be whether people are still playing it or whether the servers are just dead. Would play it on Xbox One too
Black flag and assasins creed three definitely have one.
I don't know anything about the servers at the moment, but I found some on the PS3 3-4 years ago, so it could well be that some are still alive.
Brotherhood had such a mode
These parts have an online PvP mode:
Assassin's Creed Brotherhood
Assassin's Creed Revelations
Assassin's Creed 3
Assassin's Creed 4 Black Flag
This part has an online co-op mode:
Assassin's Creed Unity
On the second question.
Multiplayer is pretty much dead.
These types of multiplayer modes were common from Brotherhood to Black Black. But hardly anyone will play anymore, even by the time the next part came out, you had noticeably noticed that hardly anyone had played the old one.
Wish there was such a multiplayer again, I spent hours with it back then, it was really great.