10 April 2018
YouTube is apparently testing a new way of showing notifications for new videos posted by channels subscribed by users. It is now showing thumbnail previews for the videos to help them stand out from other notifications and make it easier for users to get an idea of what the video is about.
You can refer the screenshot below to see what exactly YouTube is doing with new video notifications. Prior to this change, it used to show the video title, channel name, and channel's profile photo. It is now replacing that channel photo with a thumbnail preview of the video. It seems to be good in the first appearance but it can also end up eating up on the space for showing the video's title. It means you can end up with incomplete video titles if it happens to be a long one.
It should be noted that this change for YouTube is available to only a few handfuls of users selected at random. It also seems to be independent of the app version which means it is YouTube which is deciding which users will get the functionality. As a result, it is hard to tell when and if this change will be rolled out publically to everyone.