What features/instructions of x86 ISA are dead?

Mostly because it probably doesn't hurt anything to have those instructions still there, or at least the cost of having to deal with the lone enterprise customers that still use the instruction in their legacy apps is bigger.

There are a whole lot of gates available for use: every time you shrink the processor you make space for more. I bet those old unused instructions don't really occupy a lot of them, and perhaps some of them are only implemented in microcode in terms of actually used instructions. There is no pressure to make semi-dead instructions fast.

I am not an expert here but I'm semi-confident my answer is accurate. Please correct me if I'm wrong.

Some of that did actually happen with AMD64, so dropping 32bit support would be a step in that direction.

Almost no one else is in Apple’s position of vertical integration though, except perhaps for the console makers.