Now that we've finished https://jira.mongodb.org/browse/SPM-3160, per the scope, we wish to add this new syntax to the stable API once we've branched for 9.0. As such, this ticket tracks the work to add this new syntax to the stable API (e.g. by removing https://github.com/10gen/mongo/blob/6e1060533409184c09e67b3dbe665b51ad2ff41e/src/mongo/db/pipeline/expression_map_reduce_filter.cpp#L94-L98)