Unique index rebuilt as part of mongodb 4.2 startup repair/recovery will  have either kDataFormatV2KeyStringV1IndexVersionV2 (8) or kDataFormatV1KeyStringV0IndexVersionV1 (6) since FCV is uninitialized at that point of time.  To be noted, we initialize FCV only after rebuilding the index which is over here. As a result, we break the assumption mentioned here. Also, if we upgrade/restart the node with mongodb binary version 4.3+, we would crash the server as part of startup index sanity check (Sanity check got introduced only from mongodb 4.3+).