Since a dedicated MCU is the recommended solution, it doesn't need interrupts, inasmuch as it has no other tasks to perform.

The timer can serve to generate an interrupt every 20 ms or so, and, if there is a keypress detected on, say, three, successive scans, the key closure is reported once it has gone away. There are, of course, some anomalies that have to be tested-for, e.g. multiple key closures.

Serial communication can be "bit-banged" in order to communicate with the main processor.

Since these tasks can be interleaved and operated concurrently in the foreground, there's no need for interrupt-driven anything in the slave MCU.

ISP is quite slow, as compared with parallel programming. If this is intended for production, that is a valid concern.

RE