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Once you've configured a security key on a Google account, it's entirely possible to remove phone-number based account recovery -- and strongly recommended for at-risk users. See step 21 et seq. in the Tech Solidarity security guide for people working on political campaigns:

https://techsolidarity.org/resources/security_key_gmail.htm

For no particular reason, Google does require you to temporarily configure phone-number based recovery while configuring other 2FA options -- but once they're set up, you don't need the phone number anymore.