A lot of those sites will let you just reset password, send an email to the Google account's associated gmail, and then you're done. I suspect that it isn't thought through as an official feature in such cases, but it does often work.

Additionally, if you signed up with an email and password, you can often just sign in via SSO with a matching email address for convenience and still have the underlying password remain working. Again, possibly not an intentional feature (but rather something like using the email reported via SSO or via email verification as a primary key).

You can change the MX records of the domain to point at Fastmail (or some other provider) without losing the ability to log into associated "Google Workspace" accounts.

I switched away from Google and haven't had a single issue going from 'login with google' to a regular email/password FWIW. Every site has offered some kind of painless recovery.

Same; I was a sucker for GitHub SSO as well. I have no idea how to unwind it all, but (like Google SSO) it certainly had its benefits while I was using it. I remember there was an “open” SSO provider - OpenID maybe - back in the 2000s when a federated identity made interacting with blogs and such much easier. It’s the same with GitHub today, with various 3rd parties who need access to your code. They don’t have a “give me an git+ssh url” and so you almost need GitHub.

Hmmm, that is an interesting pickle. Some are probably fairly easy to swap over but it is so dependent on each service.

Alas, short term convenience can lead to long term troubles.

i’m not sure i’ll ever delete my google accounts, but my goal was to stop using them. if someday an email comes in, it’ll still get auto forwarded, and if i need to login, i’ll have access to it.

Make a new account then?