Why did my Mail app stop working on Big Sur?

My Apple email stopped working. I spent two hours on the phone with AT&T with no results. I then called Apple. The tech was very nice and knowledgeable. He believes Apple mail is no longer compatible with Big Sur. I don't want to upgrade because a few programs I have are not likely to work and i'd have to upgrade at a steep costs. I can log on to ATT or Yahoo to view email, but it's just not as convenient or easy to read. I just want to verify that it is correct that Apple Mail and Big Sur are not compatible. Thanks.

[Re-Titled by Moderator]