  We numerically study out-of-equilibrium dynamics in a family of Heisenberg
models with $1/r^6$ power-law interactions and positional disorder. Using the
semi-classical discrete truncated Wigner approximation (dTWA) method, we
investigate the time evolution of the magnetization and ensemble-averaged
single-spin purity for a strongly disordered system after initializing the
system in an out-of-equilibrium state. We find that both quantities display
robust glassy behavior for almost any value of the anisotropy parameter of the
Heisenberg Hamiltonian. Furthermore, a systematic analysis allows us to
quantitatively show that, for all the scenarios considered, the stretch power
lies close to the one analytically obtained in the Ising limit. This indicates
that glassy relaxation behavior occurs widely in disordered quantum spin
systems, independent of the particular symmetries and integrability of the
Hamiltonian.
