21700 batteries are becoming more common. Cordless tool manufacturers like Milwaukee and De Walt use 21700 batteries in some of their battery packs. I also have several Olight flashlights that use 21700 batteries. They typically have a 5000 mAH capacity vs. 2200-3000 mAH for the 18650.
21700 batteries are not same as Tesla 2170s as I understand it. Hard to find Tesla’s specs...different chemistry?
Last bit of news from Tesla was this summer about the 2170s being in full production, but only for EVs and maybe PowerWall solar storage packs.
Those flashlights using 21700 are pretty pricey....