  We examine the experimental and theoretical electron-energy loss spectra in
2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not
exhibit the regular positive quadratic plasmon dispersion that would be
expected for a normal broad-parabolic-band system. Instead we find a nearly
non-dispersing plasmon in the momentum-transfer range $q<0.35$ \AA$^{-1}$. We
argue that for a stoichiometric pure 2$H$-NbS$_2$ the dispersion relation is
expected to have a negative slope as is the case for other transition-metal
dichalcogenides. The presence of Cu impurities, required to stabilize the
crystal growth, tends to shift the negative plasmon dispersion into a positive
one, but the doping level in the current system is small enough to result in a
nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon
dispersion is not connected with the existence of a charge-density-wave order
in transition metal dichalcogenides.
