The documentation of services.xserver.libinput.additionalOptions says:
services.xserver.libinput.additionalOptions Additional options for libinput touchpad driver. See libinput(4) for available options.";
So I set it to
additionalOptions = ''Option "Ignore" "on" ''; to disable the touchpad. However, in 20.03 this doesn’t seem to affect anything, probably due to #72774. Is there a way to disable the touchpad while keeping the trackpoint enabled?