YouTube is one of the most used apps on my Shield TV; it's what my husband and I often go to when we want to catch up on videos from our favorite channels. However, two missing features have always annoyed us about the app on Android TV: one is the lack of playback speed controls, and two is the complete absence of ways to add videos to my Watch Later playlist or other lists. The latter functionality is finally rolling out now.
When browsing videos on YouTube on Android TV (and possibly other TV platforms), you may notice a new "Click and hold for more options" notice below videos. This is to point out how you can access the new feature. Do as it says and you'll get a menu on the left side that lets you save the video to a specific playlist or to your Watch Later list. On the home page, you'll also see an option to signal you're not interested in the recommended video.
Left: "Click and hold" notice below videos. Right: Options in the left side menu..
The ability to save videos to your lists is also accessible when watching them, under the overflow menu on the bottom left. A new save icon should let you add videos to whatever playlist you want.
If you don't have the option to click and hold on a video or to save it, don't worry. Google likes to roll out changes as server-side updates so it'll take a bit for everyone to have them.