Quoting karl@??? (karl@???):

> PC/AT and PS/2 have the same protocol and electrical spec. except they
> have different connectors. The protocol is bidirectional.
>
> PC/XT has the same pinout and connector as PC/AT but not the
> same protocol and they won't work together.
> The protocol is keyboard -> pc only.

Around 2000, someone brought a naggingly familiar machine to one of my
local LUGs for help doing Linux installation. I poked around it a
couple of minutes and finally recognised it with a laugh as an XT clone.
I congratulated the owner on the fact that it was still functional even
though it was 15 years out of date -- and explained that, no, we'd not
be installing a Linux distribution (though the owner could look into
ELKS, http://elks.sourceforge.net/, if the better alternative of finding
a more-modern giveaway machine somehow did not appeal).

Other than that, the only place I've seen a PC/XT keyboard since the
1990s is the Computer History Museum, so thankfully those are long gone.