I see your point. Maybe we don't allow --check with --initdb at all?
also mention that -B flag points outs to target version correctly since there is a risk of doing an initdb with the wrong version. If -B points to an older version, pg_upgrade may create the wrong cluster.
I'm not clear what pointing this out for --initdb only accomplishes. A wrong -B is going to cause problems regardless of the mode.
And the --initdb behaviour will introduce additional risks during the upgrade. During the initdb the settings discovery connections are allowed on database level and we don't have a guarantee that checkpoint and vacuums won't start. This needs to be documented clearly or we should consider eliminating them.
Eliminating what exactly? And why would a vacuum or checkpoint cause problems?
