With T204732 and T189808 in place, MediaWiki core has support for undoing and reverting revisions with multiple content slots (Multi content revisions), and also revisions with "non-editable" content in the main slot (such as wikibase entities).
This means that Wikibase most likely no longer needs to provide this functionality via SubmitEntityAction by overriding undo https://github.com/wikimedia/Wikibase/blob/master/repo/includes/Actions/SubmitEntityAction.php#L83

It's possible that some extra logic is needed, and this will need to be evaluated when the code is looked at, for example we recently touched this in T97146: [Bug] Fatal Wikibase PatcherException from EntityContent.php when undoing edits

Notes:

• "non-editable": This means content that is not editable by default in MediaWiki (so anything that is not just text / wikitext for example. Wikibase entities do not allow "direct" editing, only editing via our special APIs.

Acceptance criteria ⛺✨ :

• Reduce the "custom code" in the SubmitEntityAction::undo method, making use of what MediaWiki core provides where possible