Won’t argue the point of
But, I, and many others Mill and Drill our own PCB’s thus, we can (and do) put whatever we want on the layer that makes sense for the project at hand. As a business, I’ve farmed out Multi-Layer jobs that were milled and drilled (I don’t make my own multi-layer boards).
I generally put NPTH holes on the Edge_Cuts layer and Route them with the same 1.5mm End-Mill Bit that I use for routing the board’s shape. Sometimes I use them as a Drill-Bit…
JLCPCB specifies the smallest Hole for NPTH as 0.50mm and to be put on the Mechanical Layer (i.e., the Edge-Cuts layer).
They also specify the minimum Drill-Hole size of 0.20mm
If you think about it, why might they specify Two different minimum hole sizes (one for Drills and one for Mechanical (meaning, to be routed)? Obviously they intend to Route the 0.5mm hole and Drill the 0.20mm.
Additionally, they spec minimum Non-Plated Slot dim’s of 1.0mm on Mechanical layer, thus, they must be using a small mill-bit. They do not charge extra for this (small drilling/routing).
Images below from JLCPCB