Parallel port ftw

I’m dirt poor so all of my tech is ancient.

Many/most of them can just use a DIN to PS/2 and then if needed a PS/2 to USB adapter. Unless you have one of the more exotic old connectors (SDL, ADB, etc), that is usually all it takes.

There's a couple options from before the ps2 standard, but I'd guess it's a 5-pin DIN connector on an AT keyboard (e.g. IBM model M). You'll need to know more about the keyboard to make a determination, but you might get lucky with a passive adapter to PS2 and chain to USB. Before the ps2 standard, there was more variety between manufacturers in both signaling and connectors.

It's probably a DIN plug. They made DIN to PS2 adapters

My newest computer (custom mini-ITX with an AMD 5700X3D, 64 GB RAM, Radeon 9070XT, 4 TB NVME, etc.) is still compatible with the keyboard from my first computer (a Tandy 1000 RLX HD with an Intel 80286, 1 MB RAM, VGA graphics, 20 MB HDD). Manufactured three decades apart, but they both have PS/2 ports!

I still have that Tandy keyboard; I should dust it off and plug it in.

If it's a model M there's a guy on eBay that sells a model M USB cable.

This keyboard reminds me a lot my previous keyboard, it was PC/AT - plugged it in through a PC/AT -> PS/2 adapter.

Well, you could get this to go to PS/2: https://www.cableleader.com/6-din5-female-to-mini-din6-male-at-to-ps-2-keyboard-adapter-cable.html

Then this to get from PS/2 to USB if you need to: https://www.cableleader.com/usb-type-a-male-to-ps2-6-pin-female-converter-for-logitech.html

Absolute worst case, open it up, rip out its guts and replace with an Arduino set up for HID mode.

I don't speak from personal experience, but it's my understanding you can do this with an active (as opposed to passive) ps2 to USB adapter. Should be compatible with pretty much any ps2 keyboard.