Opportunistic reads against replica-sets was originally introduced in v4.2 as part of PM-856. This feature allows mongos to direct secondary reads to any secondary with an active connection, instead of waiting for a new connection to a specific secondary.
Under certain workloads, as described in SERVER-53853, opportunistic reads may trigger construction of unnecessary connections to mongod processes, which would regress the overall performance.
This ticket should introduce a new feature flag to disable opportunistic reads in v4.2 by default. In particular, the following tickets from PM-856 introduce new functionality that we might want to guard with the feature flag:
SERVER-41132, SERVER-41133, and SERVER-41134.