@NewUser That's not accurate. You're misinterpreting the microG toggle for FCM as an overall toggle for whether apps can use it. The toggle for FCM doesn't work correctly because Google Play or a spoofed implementation being present results in apps using it for push instead of their own push implementation. If you try to use Signal alongside it with the toggle disabled, Signal won't have working push notifications. It does not change the fact that other apps in the same profile can see it and use it. It's only a toggle for one specific feature.
DivestOS still gives it the privileged ability to pretend to be Google Play despite it not implementing the same security checks, which introduces serious security flaws and would not meet the standards for GrapheneOS.