Add maintenance_io_concurrency to postgresql.conf.sample.
New GUC from commit
# - Asynchronous Behavior -
#effective_io_concurrency = 1 # 1-1000; 0 disables prefetching
+#maintenance_io_concurrency = 10 # 1-1000; 0 disables prefetching
#max_worker_processes = 8 # (change requires restart)
#max_parallel_maintenance_workers = 2 # taken from max_parallel_workers
#max_parallel_workers_per_gather = 2 # taken from max_parallel_workers
