My Roku premier + is 2.20. Build 110005156 of the YouTube app and does not allow variable speed playback.
My Roku Express 4K+ is 2.22 build 110005090 of the YouTube app and I have variable speed playback.
Both devices show up to date. I spent a lot of money on my Roku premier + so why are the apps not updated?
You would have to ask Google. They own YouTube and provide the channel for Roku devices. We really have no way of knowing why they have different version on different devices. But I can say that other channels have shown the same thing. For example, Netflix has shown different channel version numbers on different hardware in the past. I don't use NF now, so I can't say if the numbers match now or not.
Just don't expect Google to give you a straight answer... 😄
As far as I can tell playbackSpeed is an undocumented feature of some Roku devices. If it's not on one device then that device probably doesn't support it. I'm not sure how Google ascertains what devices support it because, as I said, it's not documented anywhere that I can find. It could have something to do with the version of the channel/app, but I'd be more inclined to believe the device that doesn't show the option doesn't support it.