digital connection, where it’s literally impossible for the cable to make a difference to the sound quality
Digital isn’t magic. Lower-quality cables can very much make a difference on digital connections, including digital audio, although the effects are very different from analogue signal degradation. Granted, for the low bitrates required for audio you’d have to have a really bad cable/connector. As long as you are above a certain quality threshold, it doesn’t matter, but with surface corrosion you may end up with marginal signal levels or degraded signal edges causing more bit errors. What that means depends on the type of protocol and the kind of error detection and error correction. Best case is a very good error correction, and nothing happens. But it may lead to slower transfer speeds due to retransmits, dropouts in real-time connections, or worse.
Less than perfect conductivity or mismatched impedance may also limit the bandwidth, cause reflections, and other nasty signal degradation. It is no joke that some cheap HDMI cables cannot reliably transmit 4k signals, and the higher-quality ones generally have gold-plated contact surfaces for good reason.
