  Y3Cu9(OH)19Cl8 realizes an original anisotropic kagome model hosting a rich
magnetic phase diagram [M. Hering et al, npj Computational Materials 8, 1
(2022)]. We present an improved synthesis of large phase-pure single crystals
via an external gradient method. These crystals were investigated in details by
susceptibility, specific heat, thermal expansion, neutron scattering and local
muSR and NMR techniques. At variance with polycristalline samples, the study of
single crystals gives evidence for subtle structural instabilities at 33K and
13K which preserve the global symmetry of the system and thus the magnetic
model. At 2.1K the compound shows a magnetic transition to a coplanar (1/3,1/3)
long range order as predicted theoretically. However our analysis of the spin
wave excitations yields magnetic interactions which locate the compound closer
to the phase boundary to a classical jammed spin liquid phase. Enhanced quantum
fluctuations at this boundary may be responsible for the strongly reduced
ordered moment of the Cu2+, estimated to be 0.075muB from muSR.
