The spin-1/2 antiferromagnetic Heisenberg model on a kagome lattice is geometrically frustrated, which is expected to promote the formation of many-body quantum entangled states. The most sought-after among these is the quantum spin-liquid phase, but magnetic analogues of liquid, solid and supersolid phases may also occur, producing fractional plateaus in the magnetization. Here, we investigate the experimental realization of these predicted phases in the kagome material YCu₃(OD)_6+xBr_3−x (x ≈ 0.5). By combining thermodynamic and Raman spectroscopic techniques, we provide evidence for fractionalized spinon excitations and observe the emergence of a 1/9 magnetization plateau. These observations establish YCu₃(OD)_6+xBr_3−x as a model material for exploring the 1/9 plateau phase.