Xbox Accessories App will soon let players map keyboard keys to controller buttons | VGC
(www.videogameschronicle.com)
Funnily enough, this is the one thing Steam Input doesn't actually support. It lets you map controller inputs to m/kb or other controller inputs, but mapping m/kb to whatever else isn't supported at all. I get why it works that way but a bit of a shame, really.
Edit: The headline confused me but according to the article it does just mean controller > keyboard mappings, so never mind