The experimental-watch-progress-notify-interval flag specifies an interval
at which etcd sends data to the kube-api server.
It is used by the WatchBookmark feature which is GA since 1.17.
It will be used by a new WatchList feature which is Alpha since 1.25
In addition to that the feature was graduated to GA (non-experiment) in etcd 3.5 without any code changes
81dec8bc6a