shardCollection should not be allowed to run in a mixed-FCV cluster, because it can result in the collection that is being sharded not having a UUID stored in config.collections on the config server:

1) setFCV="3.6" called against a mongos, which forwards _configsvrSetFCV to the config server
2) configsvrSetFCV finishes generating UUIDs for existing sharded collections
3) shardCollection is called
--> the FCV on the config server has not been set to 3.6 yet, so shardCollection will not ask the primary shard for a UUID
--> and even if it did ask, the primary shard may not have had setFCV called on it yet, and so would not return a UUID
--> so an entry for the newly sharded collection will get written to config.collections without a UUID