In mongodb/specifications@747b748, the rules were applying a read preference for aggregations using $out and $merge was changed. Previously, drivers were instructed to attempt using the read preference and only fall back to a primary if a pre-5.0 secondary was selected.
This was changed to require drivers to always disregard the read preference if there is any evidence of a pre-5.0 server. If there are either no available servers or all servers are 5.0+ (or load balanced, where we assume 5.0+), drivers can utilize the read preference.
Spec tests have not been changed, as this clarification really only changes behavior for mixed version clusters or a replica set consisting of a single pre-5.0 primary, neither of which are tested.