The new notifications system replaces the earlier Google+ notifications system with YouTube-only notifications. The updated experience puts the focus on your subscriptions and sends a new notification every time a channel you are subscribed to uploads a new video if you have enabled notifications for that channel.
It is not clear if this new notifications panel will show comment notifications for content creators as well. However, as it seems it looks it will just service subscriptions. The settings icon directs users to the notifications section of the settings of YouTube where users can enable/disable notifications on a channel-by-channel basis.
The test is rolling out yet and will not appear for all for now. But in case you see this YouTube notification and don’t like it, it is possible to return to the previous version. Simply clear the cookies in your browser and that’s all you need to do to go back.