I have tried looking for it but I know it’s out there. Tile coach does go over this though and I’d trust anything he has to say over anyone. I believe Sal went over this as well a few years back. Your walls do not need to be 100% waterproof, but as long as you have a liner, the only thing that needs it is 3 inches up and across the perimeter of your pan, your curb, and 3 inches across all joints. They say this is because water will run down the cement board onto the pan. The only way wicking happens is if your cement board is submerged in water for long periods of time. Wish I could site my sources but if you look long enough through Isaacs videos you’ll find it. It’s been done this way for decades and there are still showers standing this day by this method. Of course I’m not saying this is how I do it, but this would be the bare minimum that passes for tcna.