  We update constraints on the amplitude of the primordial trispectrum, using
the final Planck mission temperature and polarization data. In the squeezed
limit, a cosmological local trispectrum would be observed as a spatial
modulation of small-scale power on the CMB sky. We reconstruct this signal as a
source of statistical anisotropy via quadratic estimator techniques. We
systematically demonstrate how the estimated power spectrum of a reconstructed
modulation field can be translated into a constraint on $\tau_\mathrm{NL}$ via
likelihood methods, demonstrating the procedures effectiveness by inferring
known $\tau_\mathrm{NL}$ signal(s) from simulations. Our baseline results
constrain $\tau_\mathrm{NL} < 1700$ at the 95\% confidence level, providing the
most stringent constraints to date.
