The answer is - it depends.
If you are using Kerdi membrane plywood would be acceptable. Actually Pine sheathing plywood is the common type I have seen used for benches and steps in showers by my tile guys.
I personally wouldn't use PT plywood. PT is perfectly acceptable to use in a home but its purpose is to handle water/moisture that should be common to the situation. Basement outer wall framing is a perfect example - and bottom plates is a requirement in most areas. However I have dealt with PT dimensional lumber and plywood and there is a significant chance of warping - even if you get your stuff from a good lumber yard. If big box you have a 80% chance it warps unless you secure its shape. The point is the last thing you want is a bow in the plywood on day 4.
The lumber used to frame your shower should never get wet. So you don't need PT. And even if your shower framing got wet, that would be the very least of your worries. Meaning get quality wood but not PT.
So I would answer that this should be plastic plus plywood with kerdi membrane over or it should be hardiboard plus some sort of roll-on waterproofing like redgard (I do not plastic behind hardiboard as I feel it doesn't weep so putting plastic would cause an area that retains water that doesn't dry which will weep down and destroy whatever is below it).
Note: Make sure you slope your bench in (doesn't need to be dramatic). The back of the bench is the most vulnerable part of the whole shower. I would make sure my waterproofing is perfect there. You don't know how many benches I have seen with no slope or sloped towards the wall.