SERVER-83663 removed the server's notion of authorization schema version. Since that schema version has not been updated since MongoDB 3.0 (and MongoDB 8.0 simply fails to start/throws exceptions if it sees a value below 5/ schemaVersion28SCRAM ), we decided that there is no need to continue reading this value for upgrade/downgrade purposes. Future changes to the authorization schema (e.g., any changes to the user or role document's schema) will be gated behind FCV.

To retain backwards compatibility with lower binVersion servers and any downstream entities that check the authorization schema version, we should continue writing the authorization schema document during FCV downgrade if there are any user or role documents on-disk at the time of downgrade.