  We develop a microscopic theory for plasmon excitations of
graphene/${\alpha}$-RuCl${_3}$ heterostructures. Within a Kondo-Kitaev model
with various interactions, a heavy Fermi liquid hosting flat bands emerges in
which the itinerant electrons of graphene effectively hybridize with the
fractionalized fermions of the Kitaev quantum spin liquid. We find novel
correlated plasmon bands induced by the interplay of flat bands and
interactions and argue that our theory is consistent with the available
experimental data on graphene/${\alpha}$-RuCl${_3}$ heterostructures. We
predict novel plasmon branches beyond the long-wavelength limit and discuss the
implications for probing correlation phenomena in other flat band systems.
