Let's define "still in use," because that's the heart of the question. Technically speaking, I'm sure someone could dig an Apple IIc out of storage somewhere and successfully run a game on it. Hell, someone out there might be doing that right now. To me, that anecdotal experience doesn't translate into "in use."
I'd define "in use" to mean still in productive service to a group of people or institutions significant enough to represent a niche market.
For that, I'd probably look to things like avionics software in certain airplanes, navigation systems on old ships, firmware in medical devices and diagnostics, or perhaps to inventory management systems in old warehouses.
And who knows what firmware we loaded onto our Cold War-era ICBMs, for that matter? Or what dusty, backwards-compatible systems we would need to maintain control over them? Would anyone like to play a game? :)