How to configure touchpad working area size/coordinates/margins?
System settings is lack off touchpad working area customizations, where you able to fit the touchpad area to the screen, without untapped areas.
I have dead margins of about 1 cm on the left, top and right of the touchpad and I have no idea how to make it sensitive.
This is a driver bug that can only be solved in the Libinput driver itself. I’d recommend reporting it at Issues · libinput / libinput · GitLab.
Impossible to fix, they said. It’s done on purpose and won’t be fixed, I guess 
I think you may have misunderstood; the problem is not caused by Libinput’s palm rejection feature, but by the fact that your touchpad specifies dead zones around the edges.
Well, I believed in the power of llinux that disabling it wasn’t so impossible.