Note that this "OAuth2" support is only for gmail. OAuth 2 is not really an open protocol like OAuth (1) was. It's just something the megacorps shoved down the IETF's throat and every implementation is different enough to need different software. OAuth2 is not good for the health of the internet and will lead to even more lock in. When Google disables imap at the end of the month for gmail (only allowing OAuth2+imap,…
I don't think many email clients support OAuth2 (Thunderbird does, but that could be Gmail specific?) but the concept isn't inherently bad, in my opinion.
I don't really see the problem with OAuth2 itself, the email space is just very very slow at accepting new standards. Dovecot and UTF8 email addresses still don't play well together, for example. The lack of proper on-the-fly application registration for mail servers is annoying, but supporting them should be easy enough if the mail service isn't run by complete doofuses.