Sort of. Google has slowly migrated all essential services into closed source libraries they control.

This isn't quite true. My GrapheneOS phone isn't lacking any "essential service." The only issue is that some apps distributed through the Play Store (or an alternative frontend like Aurora) that depend on proprietary Google libraries won't work. But this is a problem that rests with the developers of the apps, not AOSP per se.

Been using AOSP without Google Mobile Services for a decade now (LineageOS and GrapheneOS) without needing those "essential services"

Is Google going to require that device makers provide unlocked bootloaders?

Tbf I haven’t heard any news that Alphabet is requiring all sellers that paid off phones to be able to change to AOSP.