I used to work for Twitch and built the Custom Live notifications for streamers. It was a relatively straightforward change where we just changed the payload of what the streamers wanted the iOS, Android, email notifications to show. There were some behind-the-scenes work where there is actually a language/curse word check and decisions on if we needed to translate the copy to the receivers local language.

The measurable change was a 30-60% increase in the notification CTR and resulted in hundreds of millions of incremental hours watched.