After SERVER-81188 a shardCollection request is parsed using shardsvrCreateCollection parser. Considering shardsvrCreateCollection has a bigger set of fields  compared to shardCollection (as it includes the fields of the create command) the user could unintentionally set unsupported fields that belong to the create command. The consequence are that if the collection does not exists yet, we might allow a creation of an invalid sharded collection (like a capped sharded collection)