The set neck models are great for slap - they naturally have a more pronounced midrange focus, craved by most slappers. It's not impossible to get similar sounds from the neck throoughs, but it isn't as automatic as the set necks.
The space between the neck pickup and the end of the fingerboard has changed over time. The earliest basses didn't even have a truss rod cover. Of course, nobody slapped back in 1972 either. Now that playing styles have changed, the neck pickup has gradually moved back towards the bridge to allow more room for slapping, plus has the benefit of making it a little less bassy sounding, which can be a bit boomy when slapping.
You can change the placement of the neck pickup on any model for no additional charge - just specifiy the distance between the front edge of the neck pickup and the end of the fingerboard. This is Alembic, and nearly every specificaion can be changed to suit your needs.