  Monolayer B$_2$S has been recently unveiled as a desirable honeycomb
monolayer with an anisotropic Dirac cone. We investigate the
Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction, between two magnetic
impurity moments in armchair-terminated B$_2$S nanoribbons in the presence of
strain and staggered sublattice potential. By using an accurate tight-binding
model, we firstly study the electronic properties of all infinite-length
armchair B$_2$S nanoribbons (ABSNRs), with different edges, in the presence of
both strain and staggered potential. The ABSNRs show different electronic and
magnetic behaviors due to different edge morphologies. The band gap energy of
ABSNRs depends strongly upon the applied staggered potential and thus one can
engineer the electronic properties of the ABSNRs via tuning the external
staggered potential. A complete and fully reversible semiconductor (or
insulator) to metal transition has been observed via tuning the external
staggered potential, which can be easily realized experimentally. A prominent
feature is the presence of a quasiflat edge mode, isolated from the bulk modes
in the ABSNRs belong to the family $M=6p$, with $M$ the width of the ABSNR and
$p$ an integer number. The position of the quasi-flatbands(QFBs) in the energy
diagram of ABSNRs can be shifted by applying the in-plane strains. At a
critical staggered potential, for nanoribbons of arbitrary width, the QFB
changes to a perfect flatband. The RKKY interaction has an oscillating
behaviour in terms of the applied staggered potentials and width of the ribbon,
such that for two magnetic adatoms randomly distributed on the surface of an
ABSNR the staggered potential can reverse the RKKY from antiferromagnetism to
ferromagnetism and vice versa. Our findings pave the way for applications in
spintronics and pseudospin electronics devices based on ABSNRs.
