While we are unable to support OAUTH2 with pop3 (due to upstream dependency ruby/net-pop#16), we are adding the support for mail pollers plugin. Doing so, it would be possible to write a plugin which then uses other ways (microsoft graph sdk for example) to poll emails from a mailbox.
The idea is that a plugin would define a class which inherits from Email::Poller and defines a poll_mailbox static method which returns an array of strings. Then the plugin could call register_mail_poller(<class_name>) to have it registered. All the configuration (oauth2 tokens, email, etc) could be managed by sitesettings defined in the plugin.
Alessio Cosenza