I think it will be for new installs only, however there are some related specifications on the verification page: https://developer.android.com/developer-verification/guides/android-developer-console

And it seems like its even worse: now everyone who wants to make an app for global usage needs to pay Google 25 USD for the developer account even if they don't publish on the Google Play! https://developer.android.com/developer-verification/guides/android-developer-console

Is there any info on how this would be enforced? Will it be enforced on the system level or with Play Protect or something similar?

2 as the first one can be mitigated by a network update to some extent