openCollectionAtLatestByNamespaceOrUUID is only ever executed when there's a commit pending entry. A commit pending entry implies there's a MODE_X holder (or that the collection is only accessible to a txn creating it). So there can be no concurrent CRUD ops changing multikeyness.

Before SERVER-125391 it was possible for that path to find that even the commit pending instance did not match what was in storage due to multikeyness mismatch. So there was logic to create a compatible collection from storage as a last resort.

SERVER-125391 fixed index builds to stop reusing the same multikey CRUD path that deferred updating the in-memory state to onCommit, and ensured commit pending entries match what is in storage. So the above logic should no longer be necessary.