Should I apply waterproofer on the subfloor beneath a shower pan?
In such a high humidity environment, I'd remove the existing sub-floor and put in pressure treated plywood in the entire room.  Not out of concern for spilled water but because it's a steam shower, so your overall bathroom is going to become a playground for mold.



Update

Based on the comments to this answer, updated answer:

Since you're waterproofing the entire room, its not essential.  However, it's not a BAD idea to do it anyway - and I'd do the entire room not just the floor under the pan - because an ounce of water-proofer is worth several thousand dollars of ripped out bathroom due to rot...

Just in case your kerdi leaks, that is.