nosferatu comes bundled with GSF,
That's not possible. GSF is a standalone app.
There are Google Libraries that devs can add to their apps, like for A/B testing, analytics, etc. If the app is open source it's easy to check if that is the case. If it's not open source it's a lot harder to figure out what's included.
nosferatu Still wandering if there is a way for me to verify my phone is still "degoogled"?
I think only you can decide if your phone is degoogled enough for you. I'd say no closed source Google apps is "degoogled." If you agree, you're fine.
I'd say avoiding Google libraries is not really doable for most people, so I wouldn't suggest trying it unless you're crazy dedicated.