After a reboot, my keyboard suddenly didn't work in Trackmania Stadium 2.
A search on the forums found this; http://steamcommunity.com/app/232910/discussions/search/?gidforum=864960354340839181&include_deleted=1&q=keyboard+not+working
This cited KB3140743 as the problem. Reportedly, this problem also exists in GTA5. And will quite possibly affect other games as well.
I uninstalled KB3140743, rebooted, and now the keyboard works in-game again.
A bit of investigation found some link between having a controller plugged in contributing to the problem, so that may be required to reproduce the bug.
It's all very well making it very hard to prevent automatic updates, but when they break things so fundamentally you have to wonder...