The oplogReplay option to the find command (and oplogReplay bit in the legacy OP_QUERY find protocol) currently must be set by the client in order to enable an optimized code path for queries looking for a particular range of ts values in the oplog. See getOplogStartHack() in get_executor.cpp.
Instead of requiring clients to explicitly enable this optimization, the query system should automatically optimize such queries, whether or not oplogReplay is set. This should allow us to mark the oplogReplay option as deprecated. The implementation can change to accept oplogReplay but entirely ignore its true/false value.