Generally the coatings are some combination of nickel and silicon, either plated or plasma-coated to the alumin(i)um cylinder wall. Under normal use, it is hard enough that it'll probably never wear off, given adequate air filtration and oil changes. But the coating is very thin, so anything that can physically damage the coating (broken ring, foreign matter (dirt) ingestion, broken mechanical bits) will tear it up, along with the usual oil starvation. At that point, you can either try to get the cylinders re-coated (various dirt-bike sources) or look for replacement cylinders. But in normal use, it is a non-issue, if properly applied during manufacture. And since it is light, thin, conducts heat well, and integral to the cylinder, for the normal user it is much preferable to cylinder liners.