You can still just direct solder to any of the shelf controller pcb to make your own devices. There are also more resources out there than ever before for creating your own inputs.
I don't know about the Xbox side of things, but ps4/5 there's an 8 minute lockout if it's not an officially license controller[1]. Ways around this are taking an official PCB from a ps4/5 controller and soldering external buttons to it. Another is getting a pcb with a usb passthru, which you can use something like a magicboot, which you load up with controller firmware/binary file, which is not an officially supported method, you're technically supposed to dump your own from an existing controller you own, but still I don't know the real legality of that all either.
I've got an arcade stick that only works on Xbox if you daisy-chain an official controller to it. There is some kind of authentication passthrough required.
