The 1/9 magnetization plateau has recently been observed in the spin-1/2 kagome Heisenberg antiferromagnet (KHA) ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Br}}_2\left[{\mathrm{Br}}_{1-y}{\left(\mathrm{OH}\right)}_y\right]$ (YCOB), a promising quantum spin liquid (QSL) candidate. However, the 1/9 plateau has so far been reported only in YCOB, and its microscopic origin and low-energy nature remain under debate. In this study, we investigate the magnetic properties of the chlorine-substituted series ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6\left[{\left({\mathrm{Cl}}_x{\mathrm{Br}}_{1-x}\right)}_{3-y}{\left(\mathrm{OH}\right)}_y\right]$ (YCOB-Cl) and track the evolution of the 1/9 magnetization plateau across the doping series. We observe clear signatures of the 1/9 plateau at multiple nominal Cl concentrations. The temperature dependence of the plateau response evolves systematically with increasing $x$, consistent with a progressive suppression of low-energy spectral weight and the emergence of a possible effective gap scale. Complementary magnetic torque measurements further show that the magnetic oscillations accompanying the plateau are present only in the $x=0$ sample, indicating that chemical substitution strongly affects the low-energy states associated with the plateau. Taken together, our results establish YCOB-Cl as a tunable platform for exploring how chemical disorder and modified exchange networks reshape the 1/9 plateau and its associated anomalous magnetic responses.

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• Frustrated magnetism