  Structural glasses formed by quenching a melt are known to host a population
of low-energy quasilocalized (nonphononic) excitations whose frequencies
$\omega$ follow a universal $\sim\!\omega^4$ distribution as $\omega\!\to\!0$,
independently of the glass formation history, the interparticle interaction
potential or spatial dimension. Here, we show that the universal quartic law of
nonphononic excitations also holds in disordered crystals featuring finite
long-range order, which is absent in their glassy counterparts. We thus
establish that the degree of universality of the quartic law extends beyond
structural glasses quenched from a melt. We further find that disordered
crystals, whose level of disorder can be continuously controlled, host many
more quasilocalized excitations than expected based on their degree of
mechanical disorder -- quantified by the relative fluctuations of the shear
modulus -- as compared to structural glasses featuring a similar degree of
mechanical disorder. Finally, we show that the stability bound on nonlinear
quasilocalized excitations -- previously established in structural glasses --
also holds in disordered crystals. Our results are related to glass-like
anomalies experimentally observed in disordered crystals. More broadly, they
constitute an important step towards tracing the essential ingredients
necessary for the emergence of universal nonphononic excitations in disordered
solids.
