The latest beta versions that just rolled out for some of the platforms have added that feature. I'm not sure which have added it yet (iOS has, I believe tvOS has, website via Chrome has not, no idea on the others right now).
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On the AppleTV app, go to My Library and then List All. At the top of the screen you will see Upcoming Recordings, select to get the list. On the Iphone, it is on the My Library first screen. I have been using it to delete series recordings that I have already seen. When I cancel a recording, it stays in the list until I exit and return to the list, minor bug.