Actually, no keyboard exhibits "ghost keys" — phantomkeypresses.
The difference between methods of "anti-ghost"ing is whether ghost keys are avoided completely (e.g. using diodes), avoided for some key combinations (e.g. "gaming optimised matrix") or just suppressed (detected and blocked).
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.
The majority of extant keyboards at this point are probably laptop keyboards, followed by cheap rubber membrane boards. “Mechanical” keyboards are a small niche. I would guess desktop mechanical keyboards for commercial/office use are probably still bigger than the consumer market, but I’m not really sure.
I don’t have any data about the number of keyboards still in active use / still in working order, but the number of mechanical keyboards produced 2–3 decades ago was probably an order of magnitude larger than the number produced today. And while most have been scrapped by now, keyboards are pretty durable.
The past few years has seen significant growth in the number of people buying mechanical keyboards though, so you could be right that new devices make the majority.
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.