It is practically hard to find a keyboard with PS/2 interfaces. All is taken by USB. Anyway owners of older computers still use them (including myself). I see no problem with that. It is reliable and works with no problem. It happens that some older systems aren’t so much robust with USB keyboards. Joonas has discovered that his newest keyboard didn’t work using PS/2 to USB adapter. Problem was that motherboard needs a spacebar key click in order to power up.
Instead of looking for new keyboard he decided to solve this problem with DIY minimal keyboard built around Attiny2313. Generating keystroke commands and sending them using PS/2 protocol isn’t that complex and small micro can deal with this. Especially when single key press is needed. AVR generates spacebar command and sends it to PC PS/2 input using simple bit bang method. After Computer powers up an USB keyboard takes place. The project purpose is very specific, but some methods like signaling can be used in other projects where various key press are needed.