The settling time of the switch is the same, regardless of the scanning technique used.

But with a multiplexer, the output of one switch has no impact on the output of another – you can independently actuate each key. They are all essentially isolated switches with individual pull-up resistors.

The multiplexing can be done in a number of different ways, none of which need to be particularly clever. The simplest scheme is simply to connect each key directly to a GPIO on a large-pinout microcontroller, like a TQFP-144. The fewer the keys, the less GPIO you need.

This is the best way to build a keyboard, since input can be done at the clock speed of the MCU, and debouncing can happen on all pins in parallel simultaneously.

Slightly more complex would be to use a smaller MCU with a multiplexer on each input, like a 4051 or 4067. The switching time is like 20-50ns on these parts, so while it's not quite as fast as a GPIO register read, it's still pretty quick, more than fast enough for keyboards.

If you really wanted to get crazy, you could read each switch with a high speed ADC and use an ML model to debounce the keys. But at that point, you're better off just using optical switches which don't really need debouncing.

The switching time of multiplexers is on the scale of 100 nanoseconds, so it's negligible (assuming there's no extra RC components behind the multiplexer to lengthen its settling time further, a reasonable assumption for pure software denouncing).

I got a cabinet for one of my favorite arcade rhythm games, Pop'n Music, a couple of years ago. It's older Firebeat hardware so it's a non-general purpose system specifically built for arcade music games not running a general purpose OS. I was surprised by how much better it felt than the newer versions of the game which run on Windows XP with inherent extra latency in its audio playback. (Other games by the same developer have taken advantage of lower latency audio apis in newer versions of Windows but iirc Pop'n hasn't gotten off XP yet.)

99.9% of mechanical keyboards made in the last decade avoid ghost keys by using diodes, yes, primarily because they have been intended for gaming.

However, diodes are not at all universal in mechanical office keyboards, even if they use discrete switches and a PCB. If you'd look at vintage mechanical keyboards, you'll find that while many have them, many don't. Cherry even still makes some older models with Cherry MX switches and 2-key rollover.

You’re going to have to be more precise than this. Most “mechanical” keyboards from the 80s–90s do not (the Model M is not at all exceptional in this sense). I would guess most of the more recent ones (marketed to ordinary typists) don’t either.

For enthusiast-/gamer-targeted keyboards from the past few years you could be right.