If you root, unlock your bootloader or run a custom ROM, nothing changes since your device does not pass the integrity checks and Google already had a feature for developers to block apps from running on those devices.
These new additions are also entirely irrelevant for apps distributed outside of the Play Store since Play Integrity requires the app being downloaded from the store.
Also, all of these additions are entirely up to the developers to add, Google is not forcing anything.
“Security” meaning “preventing users from using the devices they own in the way they want to use them” apparently.
Silly users, thinking they actually own anything…
If you root, unlock your bootloader or run a custom ROM, nothing changes since your device does not pass the integrity checks and Google already had a feature for developers to block apps from running on those devices.
These new additions are also entirely irrelevant for apps distributed outside of the Play Store since Play Integrity requires the app being downloaded from the store.
Also, all of these additions are entirely up to the developers to add, Google is not forcing anything.
They call out making spoofing harder, although they don’t specify how.
It’s a normal thing for people to sideload apps that are distributed through the play store - that’s exactly what tools like Aurora do.
You are right that they are up to developers, but that’s the problem. It should be up to users how they run their software on their hardware.