What I'm suggesting is if you were able to pre-configure Slack to only allow logins for valid users from Google Workspace X, then even if someone creates a new workspace Y with the same domain, Slack would still be checking against workspace X. (And similar for non-Google based identity providers.)
> And people are telling you that this is not possible with the Google public OAuth API.
Yes I understand, however it is possible to integrate Slack and Google SSO in such a way that it checks that the user belongs to the correct workspace, correct? Either via the SAML integration (https://support.google.com/a/answer/6357481) or an internal Google OAuth integration? The purpose of the public Google OAuth API as opposed to the previous two options is to allow logins from non-workspace or cross-workspace Google accounts, correct?
No, there's no way to check which workspace is in use, just that the domain matches. That's the problem.
And people are telling you that this is not possible with the Google public OAuth API. When Slack asks Google's public OAuth instance if user abc@example.com is valid, Google checks with the Google Workspace associated to example.com, and returns to Slack a response saying "yes, that user is valid, here is more information from example.com". This can be the same Workspace or another one, Google isn't really telling Slack apparently.
Now, there is another field called "sub", that should be a unique ID for the Google Workspace or the specific account, but it seems that this is not always stable, per the article, so people integrating with Google OAuth don't trust it.