Mostly, modern coatings and camera design have improved. Contemporary lenses commonly use surface coatings that make dust less likely to stick, so visible dust on the front element is often removed with a simple blower.

Digital cameras have also improved: early DSLRs were much more notorious for sensor dust, while most modern cameras include built-in sensor-cleaning systems that shake dust loose automatically. Because of that, dust is less of a day-to-day issue than it used to be.

So the dust didn’t disappear, and the air isn’t necessarily cleaner—gear just handles it better. Older lenses and earlier digital bodies generally lacked these advances, which is why cleaning used to be discussed much more often.