I'm currently designing on an iPhone Plus sized canvas. Since XD is vector based, I'm wondering if my design will be scaled automatically if the new iPhone has a higher resolution screen. I don't want to have to redo everything in a hypothetical "iPhone 8" canvas but would rather XD add a setting that allows exporting at 4x or something like that.
Because you're designing with vectors, your design can be exported at any higher DPI with no loss in quality. Android actually has a 4x DPI already, so when you export assets for Android XD will automatically create images that big right now.
Newer phones sometimes have different physical screen sizes too, though. If the screen is much larger (or smaller), you probably don't want to just rescale your design as-is -- you'll want to keep individual UI elements a similar physical size and adjust the layout to make use of the increased screen real estate. This is why XD gives you separate artboard presets for, for example iPhone 5, iPhone 6/7 regular, and iPhone 6/7 Plus.