This ticket adds 5.2 aggregate operators to the stable API. The following test gives a list of operators that are now part of the stable API:
https://github.com/mongodb/mongo/commit/3cf89ad966f8b92e3f2c1884e7d671bedff23174#diff-f4f3b312ee2a1cce32f8a1c663d51980526c418c5955ea5f691b921f03559d55R19-R79

Description of Linked Ticket

A number of query features were added in 5.2, such as the 'exact top-n' operators. They are currently not a part of API version 1, but should be once a quarter has passed. This ticket tracks the work to add them in API version 1 (I believe this would be done during the 6.0 development cycle).