  While the probability density function (PDF) of the cosmic microwave
background (CMB) convergence field approximately follows a Gaussian
distribution, primordial non-Gaussianities and small contributions from
structures at low redshifts make the overall distribution slightly
non-Gaussian. Some of the late-time component can be modelled using the
distribution of galaxies and subtracted off from the original CMB lensing map
to produce a map of matter distribution at high redshifts. Using this
high-redshift mass map, we are able to directly study the early phases of
structure formation. In this work, we forecast the detectability of signatures
of non-Gaussianity due to nonlinear structure formation at $z>1.2$. Assuming
the optimal case of no systematics, we find that it is challenging to detect
such signatures in current surveys, but future experiments such as the deep
field of CMB-S4 will be able to make detections of $\sim7\sigma$.
