Isn’t this inherent to not choosing an (EDIT: external) account-recovery method?
The flip side to allowing account recovery at Google’s discretion is lessened security for everyone. (Obviously not black and white. And I agree Google should have flexibility for old accounts. But it’s an odd thing to reject a major provider over.)
AFAIK once 2FA is up, you can remove your phone number from GMail.
I know it takes time to set up a recovery account (in case the account is inactive for x months), to remove a phone number, etc. but if one's GMail is important it could be worth doing both now if it hasn't already been done.
The point is (it's not my account) that unless you religiously update the phone number in all your accounts you will at some point lose access to some of them despite being able to prove with all the other details it's you who created and used them.
Just because.
Because phone number is a very valuable identifier for the ad company.
It's a deliberate misnoner.
Another phone humber only works if you didn't lose that phone.