For game apps, I think back button is not always necessary. I see many games don’t have back button implemented. When playing games, the user might click on the back button “unintentionally”, so it’s better not doing anything. Especially for landscape mode app, it’s really weird to have back button.
But it really depends on your game style. If your game has many levels of menus, the user might like to use the back button. Especially for business app (like mine), the user would demand the back button implemented. At the beginning, I didn’t implement back button, there were tons of requests/reviews complaining about the back button is not working, so I have to implement it later.
I don’t think Google would review this. It’s really depending on you & the app. If it’s convenient to have back button feature and you have the time, do it. If it doesn’t help much and users are fine without the back button, no need to do it.
Just my two cents.