Plastic (typically PVC) drain components that are glued together are bonded and cannot be separated. This means you have to cut the drain pipe shower pan shower or to eliminate your shower basin. You can use these tools as long as the drain is available from below, although PVC pipe cuts saw. Otherwise, you can use a tiny tool.
Remove the strainer in the drain. Unscrew them if it has screws. You should be able to pry up the strainer using a flat-head screwdriver, if you don’t find any screws.
Fit the cutoff wheel. The tool has to be small enough to fit within the pipe, and the cutoff wheel has to be large enough to cut through the wall of the pipe (usually less than 1/4 inch to many plastic pipe).
Insert the rotary tool about 6 inches to the drain pipe and make a continuous cut around the circumference of the pipe from within the pipe. Use a flashlight to keep track of your progress.
Remove all the screws holding the shower pan into the framing, using a drill and No. 2 Phillips bit, after cutting through the drain pipe. You are going to need to eliminate the wall covering to get this done.
So the gap in the subfloor clears angle up the end of the pan nearest the wall, and pull the pan out of the enclosure.