The spec requires these two fields to be part of public api:

maxSetVersion: an integer or null. The largest setVersion ever reported by a primary. Default null.

maxElectionId: an ObjectId or null. The largest electionId ever reported by a primary. Default null.

These weren't publicly exposed at all before - they were instance variables only. 1492 made them into attributes, this ticket is to write documentation for them and ensure they are exposed correctly in the public api.