Google is reintroducing the background notification exit button to the YouTube app as of the latest update, which should push users forward to version 11.29. The updated version of the app which is rolling out to the Play Store as of now, will allow users to once again close out the YouTube background notification by tapping the "x" button that sits in the bottom right corner of the notification. While this is not a huge change or improvement by any means, it's certainly a change that's worthy of mentioning as it does make closing the background notification a lot simpler than before.
It is worth noting that while the button to close out the background notification is once again part of the app, this button did not appear for everyone, and would only show up for those that had a YouTube Red subscription and had background play enabled. If users had YouTube Red but did not have the background play enabled in the app's settings, the notification would not appear as the videos would not play in the background after the screen was put to sleep. Why this feature was removed in the first place is unclear, but having it does make things a little less cumbersome, and all users will have to do if they need to all of a sudden close out the background notification is wake the display and hit the "x" button as opposed to unlocking the screen and hitting pause on the video first.
It's also worth mentioning that while this feature is being added in this update, it was included before but was suddenly removed in a prior update, so this is more of a return and not a completely new change. While the reintroduction of the "close the background play" option is a change, it is a much less minor change than what users may end up seeing in the near future with a revamped comments UI, which some YouTube app users are already seeing as it's a feature that seems to be getting tested with a limited number of users. If you have YouTube Red and are eager to see this change return, the app update should be live for you.