Google's only providing the option, it's up to individual devs to enable it on their app. If the app developer has chosen to block sideloading, then they probably have a reason for going out of their way to do so. Whatever you find that reason to be should inform your decision whether or not to continue using their app.
Their reasons means nothing. It's my device. I shouldn't have to worry about an application installed on my device being policed because the developer got a hair up their ass about people downgrading.
Explain to me what would be the good reasons McDonald's has to block their app from running on a rooted device because it doesn't pass SafetyNet or whatever Google is calling it now
It's the same system, it's all part of play integrity. And that also applies to this bullshit, why does McDonald's care if I didn't install their app from the play store?