As of SERVER-29452, the featureCompatibilityVersion document can be restored after being accidentally deleted via --repair. As of SERVER-29453, it is prohibited to remove the FCV document. One danger with having this behavior is it potentially allows users to force an FCV upgrade by removing the FCV document (or using a workaround to avoid the handling in SERVER-29453) and then running --repair with a newer binary version. MongoDB 4.0 handled this risk by refusing to repair the FCV document unless all collections had UUIDs. This handling will not address the risk of someone removing the document in MongoDB 3.6 and then running --repair with MongoDB 4.2. The usefulness of this --repair behavior should be re-evaluated with respect to the risks and costs of keeping it around.