The server parameter internalValidateFeaturesAsMaster=false is set on backup nodes to allow them to accept usages of new features through the command path, regardless of featureCompatibilityVersion. This is needed because backup nodes perform an initial sync through the command path, and it is a valid state to have new features in the database even with a downgraded featureCompatibilityVersion. For example, it's a valid state to have featureCompatibilityVersion=3.2 and decimal data on a 3.4 mongod, so backup must be able to initial sync this state.

If no features require this server parameter in 3.6, we should remove support for it.