This property is a generic constant acceleration factor to be applied to the trackpoint's movement. The original idea was that this could be a generic value that can be used by other input stacks.
Realistically, the only way to *measure* the value is to check with something that moves the pointer on the screen, and that something is libinput. So now measuring the const accel depends on libinput's pointer acceleration mechanism, but that changes behaviour based on the value of const accel.
So shipping this in systemd is not good, we need to ship this in libinput instead.
As of commit 27ebea9a76fd227934b114e8369d442e6853924d we now ignore that setting completely, in favour of a range-measured approach.