EDIT: You know, after some time to cool off, Google Authenticator 2FA can still be enabled and isn’t being phased out like the less secure SMS 2FA, so it’s really not the end of the world here. The chance of permanent lockout is avoided, even if the whole Google Prompt system is still wack.
Even if you turned it back at this point, it still wouldn’t work.
This is pretty infuriating though; Google works just fine with any device that doesn’t run Android so why would they care that you’re running a custom ROM?
My guess is something less evil and more mundane: something about your number changed in their system and now they can’t send codes to it, which is why it’s grayed out. Maybe it was previously classified as a mobile number but now is classified as a landline.
Your only option, if you don’t have any backup codes, is to use that “Get Help” option they have that takes a few days and then either start carrying around backup codes, a Yubikey, or De-Google.
Hey, maybe all 3!
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To be fair, customer support is often the way hackers bypass these protections.
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As a few people pointed out, it’s only SMS thats being phased out, so using Google Auth is a superior option if you still have access to set it up. But yeah, backup codes would be great for those already locked out by accident.