The Atlassian Team is already working on Oauth2 support for IMAP (JRASERVER-63917).
This request is specifically for the ability to use a delegated user account to access the shared mailbox via IMAP Oauth2.
There is a bug in the current IMAP implementation and this would be a good opportunity to implement this via OAUTH2:
https://confluence.atlassian.com/jirakb/imap-fails-with-a3-bad-user-is-authenticated-but-not-connected-error-in-jira-server-integrated-with-office365-665420269.html

Microsoft Documentation for Shared Mailbox access via Oauth2:
https://docs.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth#sasl-xoauth2-authentication-for-shared-mailboxes-in-office-365