We discovered that actually there is nothing to do yet. Google will still support App Passwords per the following articles:
- Google Workspace Updates: Beginning September 30, 2024: third-party apps that use only a password to access Google Accounts and Google Sync will no longer be supported
- Solved: Will App Passwords work for IMAP after Sept 30 202... - Google Cloud Community
So the only thing to do if you are using SMPT/POP3 with Discourse and Google is to make sure you are using an App Password from Google.