We should check where there are still any situations in which intra-cluster communication (mongod-to-mongod or mongod-to-mongos) still uses one of the deprecated wire op codes (OP_QUERY, OP_GET_MORE, OP_KILL_CURSORS, OP_INSERT, OP_UPDATE, OP_DELETE). Any such code should be modernized to use OP_MSG, ideally as part of 5.0.0-rc0.