Also a correction on my part: it seems the AT/XT switch is more common with mechanical keyboards and there's a fair number of early/mid 90s era AT 5-pin DIN plug style boards out there that don't have it. Some of those might also be auto-switching .
(the Tandy Enhanced 101 key keyboard supposedly is, and is a variant of the Fujitsu FKB4700 series, but I'm not sure if the generic/consumer FKB4700s are auto-switching, too; some late model Tandy boards use PS/2 connectors but are still apparently XT compatible and some of their computers use the XT protocol but have that connector natively, so it's even more confusing on that end: but cool for universal keyboards with a simple passive adapter).
I've got an FKB4700 (or 4725) that's working, but haven't tried it in an XT board. (didn't know the Tandy relation when I was messing around with it and testing the Turbo XT board I recently got)
Additionally, some cases have AT/XT switch holes in them from earlier models that go to nowhere, just bare PCB exposed underneath, so you should double-check with ebay sellers if the picture is ambiguous.
I'm not sure, but I don't think many or any of the lower-end rubber dome or similar technology boards of the early 90s tend to be auto-switching, but could be totally wrong here. (there's also some fairly cheaply made/flimsy case designed boards that use discrete mechanical switches, but those seem to more often have AT/XT switches present for whatever reason, maybe the controller ICs used more typically with those types switches has something to do with it: the older, cheap keyboard types that tend to be switchable are the foam and foil ones, and while not great for typing and potentially failure-prone due to foam degradation, a lot of those seem to be fairly collectable these days even if not uncommon)
There's also some really, really nice mechanical keyboards that have been selling at somewhat reasonable prices lately (though it seems like there were bigger batches in January and February) if you don't mind a bit of extra space eaten up. The Northgate Omneykey (and a few other northgate models) have been showing up a lot at auction lately, and if you're looking for a geneuine XT and PC/AT style key layout: those tend to have both the left-hand function keys and (sometimes) top row as well. Some have a model M style cursor key T arrangement and others have a diamond cursor key layout embedded in the middle of the navigation keys. I think they all have the big, AT style enter key and split right shift/backslash arrangement)
Those might have dried up, but they might pop up again. (a lot tend to be overpriced and some dirty ones went for not crazy, but not cheap prices IMO, especially if the buyer wanted a nice mechanical board given alps switches are nice, but don't hold up well to heavy dirt: they tend to get rough/gritty/stiff feeling, though I don't think they tend to fail)
A lot of ebay listings erroneously have 'clicky' or 'mechanical' in the titles too, so be aware of that. (though if the model number or FCC ID is visible or listed, they're often easy to find more details about)
There's also plenty of non-clicky mechanical keyboards (and linear key switches were really common in the early home computer era: lots of Radio Shack/Tandy, Atari, Apple, and Commodore boards that used those along with flat-top or domed membrane or chicklet keyes on the cost-saving models). Same for a lot of late 70s and early 80s terminal keyboards, plus lots of weird/exotic mechanisms.
for example:
https://www.ebay.com/itm/Vintage-1993-Identit … er/193307504874?
I got one like that on the off chance it would be XT compatible, but there's no switch. I didn't pay near that much though and while it was loose, it's in good shape. (unusual, very quiet, very tactile, non-clacky dome with slider keys, so kind of a curiosity at least)
But searching the FCC ide quickly turned this up (after the fact).
https://deskthority.net/wiki/Monterey_K208
(turns out it's a rebrand/variant of that board)
OTOH, if XT compatibility is the only requirement (and working condition, regardless of look/feel/etc), there's probably a wider array of 80s vintage boards that are compatible that can be had for cheaper. You just get some nicer surprise unknown/mystery listings mixed in ... and lots of overpriced crap, of course. (just browse through a few different keyword searches, if you're using ebay, with 'old' 'used' 'vintage' etc ... you can try 'mechanical' or 'clicky' in there too and see if any cheap listings pop up that turn out to be something interesting)
And sometimes you'll get old rubber dome, some other odd membrane + slider design, or foam and foil based boards (or discrete mechanical contact switch boards that are known but generally unliked) that are listed as 'mechanical' or such, but actually at a reasonable price for what they are.
Foam and foil boards also have unlimited key rollover, so if there's any need for that there's a plus there. (and degraded foam still tends to register keypresses, but feels awful) I'm not sure there's much XT-compatible games or other software out there that really demands that feature, even though the original IBM model F itself features such.
The Deskthority wiki and forums seem like a pretty good source of info for this stuff. (I finally figured out what my/Dad's old boards were, too, more or less: though one or two might be unknown/unlisted variants) I suspect all 3 AT/XT boards were bought from one of the local PC/electronics surplus warehouses back in the day.
I think geekhack and vintage computer federation came up with some useful threads on some keyboard related stuff I was searching for.
Also, aside from very early Model M boards (and the Model F AT), IBM keyboards are noted to not be auto-switching or switchable.
I think this topic ended up making me geek out more over old keyboards, and certainly found out a lot more than I knew a month or two ago. (makes me sad for another thing I didn't sort through the bargain bin for at Weird Stuff when it was around ... or the freeby bin outside: I do have an old Keytronic board from there, pretty nice for a rubber dome board and better matrix mapping/rollover than my Silitek mechanical board: Apolloboy 'upgraded' from the Silitek board he'd borrowed after getting frustrated with conflicting keymaps in several Doom engine games, though that's also a problem the Model M tends to have)
I'd also have thought some late-gen XT clone system boards switched to the AT interface, but if any did they seem to be rare. Maybe the cost of an AT/PS2 compatible keyboard controller IC wasn't worth it, especially with the highly integrated nature of the late 80s era Turbo XT boards. I guess that's part of why a fair number of keyboards continued to feature XT compatible modes even into the early 90s (that and Tandy computers using it), though actual 90s vintage ones are probably rare and definitely hit or miss these days.
I suppose white-box PC builders/sellers and home builders would've liked that flexibility, too. (build an XT class system in a baby AT case with universal keyboard, potentially with an 8-bit VGA card installed, so you could do a board+CPU upgrade and opt to change nothing else) Or not even a baby AT case, but an XT clone compatible one given most baby AT boards have XT mounting hole spacing, too.
