Engineering Ticket Description:

If mongos is committed to matching the drivers Server Selection Spec, it needs to be updated to match behavior we decided on for drivers last year. See this section of the Server Selection Spec:

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#use-of-read-preferences-with-commands

mongos should obey the "$readPreference" document sent with any command, regardless of the command name. The "whitelist" from the old Read Preferences spec is superseded by the new, simpler Server Selection Spec behavior.