As per https://docs.mongodb.com/manual/reference/method/UUID/

Should be able to call UUID() in the shell with no arguments to receive back a random UUID().

This works in legacy mongo shell

In mongos this returns an error if no string parameter provided:

Enterprise > UUID()
MongoshInvalidInputError: [COMMON-10001] Missing required argument

 This capability is required in aggregation pipelines which need to generate UUIDs (e.g for data masking and later cross referencing of that value).