I designed and built one of these for a school, twenty years ago. Just used an Atmel (before Microchip bought them) AT90S2313, memory serving. Very simple assembly code. One cool thing is that there were exactly (luckily) 8 switches to monitor. So they were all on a single port and I debounced all the switches simultaneously, in parallel. We decided to use a button like this one. Today, there are also products like this.
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