  We report on a realistic and rather general scheme where noncollinear
magnetic textures proximitized with the most common $s$-wave superconductor can
appear as the alternative to $p$-wave superconductor{--}the prime proposal to
realize two-dimensional (2D) Kitaev model for topological superconductors
(TSCs) hosting Majorana flat edge mode (MFEM). A general minimal Hamiltonian
suitable for magnet/superconductor heterostructures reveals robust MFEM within
the gap of Shiba bands due to the emergence of an effective ``$p_x+p_y$"-type
$p$-wave pairing, spatially localized at the edges of a 2D magnetic domain of
spin-spiral. We finally verify this concept by considering Mn (Cr) monolayer
grown on a $s$-wave superconducting substrate, Nb(110) under strain (Nb(001)).
In both 2D cases, the antiferromagnetic spin-spiral solutions exhibit robust
MFEM at certain domain edges that is beyond the scope of the trivial extension
of 1D spin-chain model in 2D. This approach, particularly when the MFEM appears
in the TSC phase for such heterostructure materials, offers a perspective to
extend the realm of the TSC in 2D.
