After you have put down a double layer of plywood to provide the base for a custom walk in shower pan you need to cover the subfloor with a layer of tar paper in this case referred to as a vapor.
Shower pan on a wood floor plywood thickness.
Reinforce the shower pan.
A plywood subfloor under a shower also known as underlayment is vulnerable to moisture damage that can rot the wood.
Install the liner over a pre sloped bed as i explain here.
Where a rotten subfloor is left unattended the rot may extend to the floor.
When choosing your subfloor material take into consideration how the thickness will effect the overall heigh of the built up shower pan in relation to your shower curb.
A mortared shower pan allows you to custom fit a shower enclosure.
I used an unbonded mortar bed so i have a tar.
Water that gets into the mortar bed won t be able to drain out.
No matter what the building inspector says it s not a good idea to install a pan liner and then a single sloped mortar bed over it.