I'd say that's well supported, especially for the problem it's trying to solve (displaying the best keyboard for the input on mobile devices).
[1] https://caniuse.com/#feat=input-inputmode
You shouldn't find yourself in too much trouble in a browser if you add an attribute to an element that it doesn't understand though, it will just ignore it.
I'd say that's well supported, especially for the problem it's trying to solve (displaying the best keyboard for the input on mobile devices).
[1] https://caniuse.com/#feat=input-inputmode