[x0x0]

Valve would obviously keep every hardware you've ever logged into the steam client on. So if you've played a steam game on the Sunahe account on a windows box, aha, Sunahe has windows! No warning.

But a red box for graphics intensive games that says "We've never seen you on a device that can play this" is, again, a low bar. Or a windows only game for an account that is mac only.

[spdionis]

Not very useful. If you have at least once played on an old computer that you don't use anymore but which had windows installed this would break.

And before you say that steam could detect that i don't use that computer anymore, they would have to ask for confirmation and that would be unexpected and annoying in my opinion.

Generally the necessary UX would be too complex to be useful.

[izacus]

All that effort just to avoid something PC gamers did for a long time - check hardware requirements to see if it'll run on their device.

[untoreh]

Steam already records the pcs you login from and even let you make them (steam guard)