If you really want nice looking smaller icons (16x16, 32x32) you'll have to hand edit them after scaling down to get something that looks crisp.
That said I just checked some macOS system icons (get info on app, select icon, copy, create new document in Preview.app) and they don't seem to be hand adjusted any more.
I’d say hand-crafting pixel-perfect icons with different versions for extreme sizes is worthwhile (and I don’t believe in simple rasterization from vector for the same reason), but not every developer would have resources to spare for that.
