There is a support thread tracking the issue about not being able to manage playback speed using apps on devices other than Android (and only if not casting) at https://support.google.com/youtube/thread/153199802. I suggest following it for updates.
To get action on this, go to YouTube and submit feedback describing the issue and how it impacts your desire to use YouTube.
My use of YouTube is down 50% because of this issue. How about you?
If you don’t know where the feedback option is, please see the Featured Image for this post.
© Scott S. Nelson
Historical follow up: the took down the thread. I since found out it does work on Chromecast. So, essentially it is now only available in Google apps on Google devices. This is why I stopped buying Macs in the 90’s.