This has already happened! My school promised email for life, and uses Microsoft's Office365. A couple months ago they sent out emails saying if you don't update your client to use "modern auth" (which was a new term for me) I'd lose the ability to check my email.

Well Microsoft blocks the Thunderbird embedded browser, so you can't complete the Oauth2 login. I've been effectively locked out of my account since (I'm not going to use the webmail just for this account - set up a forwarding email and told everyone to use my new address).