The NetworkService::register_notifications_protocol method is a hack, as explained here.
The problem with this method is that we might have started already connecting to other nodes.

At the moment, if two nodes connect before register_notifications_protocol has been called, and if then we try to send a message on the given protocol, it will be sent on the legacy substream as a fallback.

This is however getting in the way of removing the legacy substream. cc #5670

The proper fix is to refactor Substrate to pass the list of protocols in the NetworkConfiguration, but that's a lot of work.