When a shard is removed, its ReplicaSetMonitor is removed when the ShardRegistry next refreshes for shards/mongos or when the removal completes on the config server. If a concurrent request tries to use that RSM, it will fail with ShutdownInProgress. add_remove_shards_killop.js uses a $currentOp that may target all shards while concurrently removing shards, and can hit this error, which currently fails the test.