I noticed that https://docs.mongodb.com/v5.0/release-notes/5.0-downgrade-sharded-cluster/#5.-downgrade-feature-compatibility-version--fcv- says "Ensure that no nodes have a newlyAdded field in their replica set configuration. Use the rs.conf() method on each node in your sharded cluster to verify this. The newlyAdded only appears in a node's replica set configuration document during and shortly after an initial sync."

rs.conf() would omit newlyAdded fields due to SERVER-46353. Users who would like to see any ‘newlyAdded’ fields can do so by querying the ‘local.system.replset’ collection directly.