  Using bilayer CrI$_3$ as an example, we demonstrate that stacking domain
walls in van der Waals magnets can host one dimensional (1D) magnon channels,
which have lower energies than bulk magnons. Interestingly, some magnon
channels are hidden in magnetically homogeneous background and can only be
inferred with the knowledge of stacking domain walls. Compared to 1D magnons
confined in magnetic domain walls, 1D magnons in stacking domain walls are more
stable against external perturbations. We show that the relaxed moir\'e
superlattices of small-angle twisted bilayer CrI$_3$ is a natural realization
of stacking domain walls and host interconnected moir\'e magnon network. Our
work reveals the importance of stacking domain walls in understanding magnetic
properties of van der Waals magnets, and extends the scope of stacking
engineering to magnetic dynamics.
