Do mutt and git-send-email have support for OAuth2?
Not sure, I know that newer versions of thunderbird and k9 do, but you may need remove the account and re-create it; I didn't test changing the settings myself, I just deleted the old accounts and re-created them as IMAP / oauth.
I don't know about git-send-mail, but mainline mutt has oauth2 support for imap4, pop3, and smtp.
I haven't used it but it looks like it pushes the oauth2 token refresh off to an external script.
The problem with OAuth and Google is that Google requires from 3rd party vendors verification and it's very hard to obtain. Unsurprisingly, Google rejects requests for verification without any explanation and ignores any follow up communication.
I despise oauth2 reauthentication requests in a mail client. It's insanely annoying.