Here’s an (oldish) FAQ article: Castellated edge; plated half holes in board edge
I have visited the problem a couple of times with later versions, this one was originally written for v5. The basics are still the same. Track routing may still be a small problem. Nowadays KiCad has “Castellated pad” fabrication property for pads (and then the checkbox in Board Setup), but it doesn’t help much for board manufacturing process. Basically, if a copper feature goes over the board edge, the manufacturer takes it as a hint that the edge should be plated. But you have to ask or communicate it clearly.