The pan liner is technically part of the plumbing and is always required to be installed by plumbers if your local compliance jurisdiction is actually complying with the building codes.
The bulk of my work has traditionally been done outside compliance jurisdictions and my plumbers are quite happy to do nothing in a shower but stub up a riser pipe that's not glued to the trap.
In compliance areas we've always had an agreement whereby the plumber would "install" the liner by just kinda stuffing it into the shower floor opening and tacking the top edges to.....whatever and do a flood test. My tile guy would come in, remove the liner, build a pre-slope, install the liner properly, and move on to mud the shower.
Just a way of life when you generally get a blank stare from the plumbers when you mention the pre-slope.
I've always counted myself fortunate to have been "trained" by a good tile many early in my building career.