This thing does a much better job at cleaning their nano-texture displays compared to a regular, smooth microfiber lens cleaning cloth, so there is actually a reason for it to exist.
That being said, while I haven't tested this, you can get other "suede" style microfiber cloths that I suspect are functionally the same thing as the Apple Polishing Cloth.
Can I wash this myself or do I have to send it to Apple for their proprietary laundering method?
Yeah, but did you see the compatibility list? Disappointed I'll need to move to iPhone 5 or higher to use it.
Anyone know if I can use this to wipe a OnePlus 6 device?
I'd recommend flashing the cloth with open source firmware first. Otherwise it might check a signature in the glass, find that it's non-Apple, and become scratchy.