Configuration Changes (Atlas, STAR, docs)
The ShardingTaskExecutorPoolReplicaSetMatching server parameter (see https://docs.mongodb.com/manual/reference/parameters/#param.ShardingTaskExecutorPoolReplicaSetMatching) has a new possible value, "automatic". This new value is now the default. The behavior of this new value depends on the process: on mongos, it is identical to the existing "matchPrimaryNode" option; on mongod, it is identical to the existing "disabled" option. (On mongos, this parameter determines the minimum size limit of the mongos instance's connection pools to the secondaries of the cluster's shards; on mongod, this parameter determines the minimum size limit of the mongod instance's connection pools to the secondaries of other shards in the cluster.)
Command results changes (ADL, drivers, BIC, docs):
The connPoolStats command output now contains an additional field, "replicaSetMatchingStrategy"; the value of this field describes the actual policy used by that process to determine the min size of connection pools from that server to RS secondaries in (other) shards. The policy will either be "matchPrimaryNode", "matchBusiestNode", or "disabled", with the behavior described by the existing docs for the STEPRSM parameter. Note that even if the "automatic" value is used to set the server parameter, the "replicaSetMatchingStrategy" field in the connPoolStats output will describe the actual policy being used, not "automatic." This contrasts with the output of using getParameter() to find the value of the STEPRSM parameter, which will display "automatic" if that is the value of the server parameter.