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. With the introduction of the Onewheel GT model and the subsequent announcement of selling replacement tires, Future Motion confirmed that tire swaps no longer void your Onewheel’s warranty.
It should go without saying; any modification that requires you to break the warranty seal sticker will in fact, void your warranty.
Also note that there have been reports from riders that boards sent back to Future Motion for repair with aftermarket parts have been returned with said parts being removed, and in some cases, have received refusals of repair due to the modifications. Contact Future Motion support before you send your Onewheel in for any repairs if you’re concerned with this.