That is a real problem. So far, I have always looked from a pedestrian perspective and then it would be kerb=no. Maybe it is worth mentioning the perspective in the wiki at least in connections with highway=crossing.
In case of detailed mapped areas with sidewalks and cycle-tracks as separate objects the best solution is to tag the barrier=kerb, kerb=* node at the exact position only as child of one way or end/start node of a maximum of two ways.
Please, do not use :left, :right or :both on nodes. Nodes do not have any direction. :both just lengthen the key and the other two only work in conjunction with a single parent way and lack support in editing software. If you are eager to support different values per side, cardinal directions might work.