Tried to log in to an old gmail account. Remembered my password perfectly. But it asked for a confirmation code from an old phone number I no longer have (something it never did before). No alternative recovery options.

This is why I keep shilling decentralized social recovery.

Had the same happen to me. The only option is to speak with a Google employee or ask the new owner of your phone number to help, neither of which are great options.

For consumer Gmail accounts you will never reach an employee, and even if you did, there's nothing they can do.

Only paid Google Workspace accounts have the support to be recovered by other means than your recovery options.