setup:di:compile fails after the upgrade. What is going on?

A di:compile failure after an upgrade almost always traces to one of a short list: a di.xml preference pointing at a class that was removed, a constructor argument that no longer exists, a plugin on a final or removed method, or a proxy for a deleted type. The store will not deploy until compile passes, so this is high priority. We read the compile fatal, trace it to the exact module and line, correct the type or argument, and re-run compile to prove it is clean before anything ships.