This code was added in SERVER-36718 because it was expected that a big index key inserted in FCV 4.2 causes the index validation in FCV 4.0 to fail.

validate should no longer fail because of the server running with an FCV less than the latest version.

There's also a build failure where the fuzzer creates a measurement index on a time-series collection and cannot complete a previously failed FCV downgrade, causing the hook to fail.