This doesn't really have much to do with Forza Horizon 4. As, if it's like Forza Horizon 3, you will still be able to play the game online and offline. This is what the stop killing games wants the other publishers to do.
I’d go one further: The movement can come across as whiney and impossible to please when it echoes the message in a super-blanket way in places it doesn’t make sense, like this one.
It’s like protesting government actions with “All government should be abolished so we can ALL BE FREE”.
Yeah, the only thing Microsoft could do in this case is to stop using licenced vehicles and music. And I'm pretty sure we would be hearing pretty loud complaints if that happened.
This is about the best case scenario. The game is even on sale to those who almost missed out on the game.