There should be - there's three LEDs (capslock, numlock and I forget the third one) on standard keyboards which can all be set from the OS side... and at least caps lock (I think!) can be set from Javascript, so it's essentially an 1 bit wide communications bus, three bits if it's an ordinary OS tool.
I love scroll lock still existing on keyboards. Some operating systems still use it to control text scroll for some reason (useful when you don't have `screen` installed maybe?), but I mostly use it for toggles in video games. It shows the toggle state on a physical interface, like one of those fancy programmable macro keyboards, but on commodity hardware!
There should be - there's three LEDs (capslock, numlock and I forget the third one) on standard keyboards which can all be set from the OS side... and at least caps lock (I think!) can be set from Javascript, so it's essentially an 1 bit wide communications bus, three bits if it's an ordinary OS tool.