It's happened, Marvel's Avengers is no more. Just three years after launch, the game has been delisted from all storefronts but will remain playable for anyone who picked it up before the takedown. You should still be able to play with friends, but any issues you run into won't be addressed, and there will be no more events.
Marvel's Avengers at least ended on a slightly better note than it opened on, as the price was cut by 90 percent. This netted you the Definitive Edition, which includes all of the cosmetics and DLC for free. In practice, this should help Marvel's Avengers feel like any other single-player game, rather than a live service that just had its roadmap come to an end.