I was also under the impression that it was the full modern authentication, but it can be tricky when another team is in charge of the O365 environment. I believe that the Remote Mailbox set up should still use the same modern authentication method, though. When attempting to set up Remote Mailbox, I am able to authenticate and create a Token by entering my personal ID info.
Info:
IdP config:
Token:
I checked with the O365 Manager and they've confirmed that the Azure App didn't change at all. Its the same App ID and Secret Value as it has always been.
