One reason to remove the features is if they started supoorting another format (HEVC? ProRes?) for which thir decoder doesn't yet support these operations?

Even if that were the true motivation (which I don't believe), there's a standard UI paradigm for that: grey out the control if it doesn't apply.

I'm more inclined to believe that it was a UX designer with a misplaced drive to simplify at all costs.

On Catalina at least, variable-speed constant-pitch playback is still available, just hidden. Option-click on the fast-forward or rewind buttons to access it. It jumps in increments of 0.1x.

Thanks, I didn't know that! It doesn't let you go slower than 1x speed though, which was the useful thing. I used to use it to learn tunes by playing half speed.

macOS is full of things that randomly pop up if you click something while holding a button. That UX is so hilariously bad it makes features hard to find even when you already know they are there!