Some modifications do, but most cosmetic accessories and modifications do not.
External cosmetic mods like rail guards, fenders, bumpers and tires do not void any warranty. In fact, Onewheel sells OEM accessories, so customization is encouraged. However, if you open up the board and access components with warranty seals, you can kiss that warranty goodbye. Consult the Onewheel warranty and/or contact Future Motion for further clarification, if unsure.
If you’re interesting in learning more on the topic of how riders modify their board to increase battery life and ride length, check out the customization section below.