Can debug logging be added to firestore rules functions?

You may want to look into local rules emulation using the Firebase CLI, which is a brand new feature of the CLI. You can do simple logging with the emulator with the debug() function.

However, there is no way to log anything in security rules in production. If you want to verify that your rules work as expected, you should write some integration tests for those and run your tests to make sure access is rejected or allowed according to your specifications.

Firestore rules now have a debug() function

It's still not brilliant but better than before.