Oracle does have a SPF include for their domains. https://docs.oracle.com/en-us/iaas/Content/Email/Tasks/configurespf.htm

How was someone to know they changed the recommended parameters, though? They made this change with no warning or thought to how it could affect accounts. "Surprise!"

Exactly. Then start sending thousands of emails on a new server with no warm up. Microsoft went to strict compliance on May 5th for volume senders so no surprise they were blocked. I have went back and forth with them for over two weeks to explain how email works. They closed my ticket three times as resolved.

Sad state of affairs there lately. The sagas continue.