We can confirm that Marvel’s Avengers is still coming to Stadia on September 4. There was some worry that it was not coming due to the Stadia logo not appearing in the launch trailer. We reached out to Square Enix and received this response :-
Thanks for reaching out about Marvel’s Avengers. Yes, the game is launching on PlayStation 4, Xbox One, Stadia and PC on September 4, 2020.
So while we don’t know why the latest trailer does not have Stadia listed, the game is still coming the same time as the other platforms. This is another big mainstream title coming day and date to Stadia. The next number of months Stadia gets a number of titles including the Avengers, Assassins Creed Valhalla, Star Wars Jedi Fallen and more. We also discovered a list of 19 games listed by PEGI which maybe coming to the platform.