The $slice(projection) operator can be used with an array as 1st parameter, like the $slice(aggregation) operator.

db.posts.find( {}, { com: { $slice: ["$comments", 1, 3] } } ) 

This is told by the server error when we use only 1 parameter in an array

db.posts.find( {}, { comments: { $slice: [3] } } ) 

Invalid $slice syntax. The given syntax { $slice: [ 3 ] } did not match the find() syntax because :: Location31272: $slice array argument should be of form [skip, limit] :: The given syntax did not match the expression $slice syntax. :: caused by :: Expression $slice takes at least 2 arguments, and at most 3, but 1 were passed in.

The review must be done by the server team, I've only found this undocumented supported feature.