This was a fun read. Hall effect switches especially feel like they came out of nowhere in the last couple years, but seeing them broken down like this makes the hype make more sense.
(* XBOX Controller Mods: Analog WASD Gaming Keyboard https://www.youtube.com/watch?v=gEwDImE0DU4 2012 *)
They were typically used in the more expensive models of video terminals that were used together with minicomputers or mainframes.
Personal computers used cheaper keyboards, to minimize the cost, so after everybody switched to using PCs the more expensive but better keyboards with Hall effect sensors have disappeared.
The keys with Hall effect sensors contain a special analog integrated circuit instead of metallic contacts. The IC is packaged similarly with a transistor, having 4 or sometimes only 3 pins that are soldered on the keyboard PCB.
So to make this kind of keyboard, you need a vendor for such an integrated circuit. Semiconductor IC vendors do not make such ICs, unless there is a market big enough for them.
During the seventies and the eighties, most IC vendors had such ICs in their catalog, but when the market for expensive keyboards disappeared, the production of such ICs has also stopped.
This has made more difficult for anyone to start making such keyboards, instead of other kinds of keyboards, because they must bet on selling enough of them, to be able to convince an IC vendor to make a batch of such ICs.
I have not seen any modern keyboard with Hall effect sensors, so I do not know whether they really use the kind of sensors made for keyboards that have been used originally. Perhaps they repurpose Hall sensors made for brushless DC motors, which are easily available anywhere, but they might need some additional external components in comparison with the special keyboard sensors.