I think that the update is just to scare Kid YouTubers to change into PG-13 youtubers. They are trying to change all content and make kids under the age of 13 watch Disney Channel instead of watching Kid YouTubers like DanTDM or Stampy.
The only possible reason I can come up with is to prevent kids from searching YouTube while the video is playing in the corner if that makes sense. It restricts them to the video itself as well the recommendations on the side which are all kids videos to keep them safe and away from inappropriate content. As for the notification bell being turned off, that's probably to prevent kids from making accounts and trying to avoid COPPA and it's restrictions as they tend to do that a lot. Same goes for all the other features disabled.