  A quantitative understanding of the electromagnetic response of materials is
essential for the precise engineering of maximal, versatile, and controllable
light--matter interactions. Material surfaces, in particular, are prominent
platforms for enhancing electromagnetic interactions and for tailoring chemical
processes. However, at the deep nanoscale, the electromagnetic response of
electron systems is significantly impacted by quantum surface-response at
material interfaces, which is challenging to probe using standard optical
techniques. Here, we show how ultra-confined acoustic graphene plasmons (AGPs)
in graphene--dielectric--metal structures can be used to probe the quantum
surface-response functions of nearby metals, here encoded through the so-called
Feibelman $d$-parameters. Based on our theoretical formalism, we introduce a
concrete proposal for experimentally inferring the low-frequency quantum
response of metals from quantum shifts of the AGPs' dispersion, and demonstrate
that the high field confinement of AGPs can resolve intrinsically quantum
mechanical electronic length-scales with subnanometer resolution. Our findings
reveal a promising scheme to probe the quantum response of metals, and further
suggest the utilization of AGPs as plasmon rulers with \r{a}ngstr\"{o}m-scale
accuracy.
