Multigap and high- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>T</mml:mi> <mml:mi>c</mml:mi> </mml:msub> </mml:math> superconductivity in metal-atom-free borocarbides: Effects of dimensional confinement and strain eng

Pure borocarbides suffer from limited superconducting potential due to intrinsic structural instability, requiring transition/alkali metals as dual-functional stabilizers and dopants. Here, by combining high-throughput screening with anisotropic Migdal-Eliashberg theory, we identify dynamically stable borocarbides where high-${T}_{c}$ superconductivity predominately originates from $E$ symmetry-selective electron-phonon coupling (EPC). The six distinct superconducting gaps emerge from a staircase distribution or uncoupling of EPC strength $\ensuremath{\rho}({\ensuremath{\lambda}}_{k}^{el})$ across each Fermi surface sheet, constituting a metal-free system with such high gap multiplicity. Crucially, dimensional reduction from bulk to surface strengthens $E$-symmetry EPC and enhances ${T}_{c}$ from 32 K [three-dimensional (3D) bulk] to 75 K (2D surface), a result that highlights structural confinement as a key design strategy for observing high ${T}_{c}$. External strain further optimizes the competition between EPC strength and characteristic phonon frequency to achieve ${T}_{c}&gt;90$ K. This work reveals a systematic correlation between structural dimensionality and gap multiplicity and establishes borocarbide as a tunable platform to engineer both high-${T}_{c}$ and multigap superconductivity.