I've often read that 2FA should be through an authenticator app or a physical key, not via texting a code to a phone number. Malicious sim swapping is a thing, so purposely deleting any phone number from an account should be good practice, right?

So will they also delete inactive accounts that have no phone number, but one or more phone-less 2FA methods associated?

I struck the same as described in OP a couple of years ago. Yes, regardless of other methods of contract for 2FA, I recall even when I thought I had it licked, the process reverted back to asking for a phone number to contract. I tried with a different number of browsers and eventually the latest up to date machine with the same failure.

Google started using their mobile app for 2FA on my account without me ever opting in. Now I’m afraid if I delete the Gmail app from my phone I’ll lose access to my account.

They default to authing you via a sms code usually still, even when you have an authenticator paired