  CMB lensing maps probe the mass distribution in projection out to high
redshifts, but significant sensitivity to low-redshift structure remains. In
this paper we discuss a method to remove the low-redshift contributions from
CMB lensing mass maps by subtracting suitably scaled galaxy density maps,
nulling the low redshift structure with a model-insensitive procedure that is
similar to delensing. This results in a high-$z$-only mass map that can provide
a probe of structure growth at uniquely high redshifts: if systematics can be
controlled, we forecast that CMB-S4 lensing combined with a Rubin-LSST-like
galaxy survey can probe the amplitude of structure at redshifts $z>3.75$
($z>5$) to within $2.3\%$ ($3.3\%$). We then discuss other example applications
of such high-$z$ CMB lensing maps. In standard analyses of CMB lensing,
assuming the wrong dark energy model (or wrong model parametrization) can lead
to biases in neutrino mass constraints. In contrast, we show with forecasts
that a high-$z$ mass map constructed from CMB-S4 lensing and LSST galaxies can
provide a nearly model-independent neutrino mass constraint, with only
negligible sensitivity to the presence of non-standard dark energy models,
irrespective of their parametrization.
