$near, $nearSphere, and $geoNear are not allowed in agg pipelines. This can be confusing for users as they can use these operators in non-agg geo queries. We are editing the error message to better explain why these operators are not permitted in an agg context and suggest workarounds (notably, use $geoWithin in agg pipeline if they don't care about sort). You can follow the conversation on SERVER-57932.

We would like to have documentation describing the workarounds in more detail and link to this documentation in our error message.