This is actually self enforcing for the most part. Raising kids who can behave themselves in a modern restaurant (e.g., 100+ dB crowd noise, large open spaces, and hour-long waits) involves a certain amount of luck. It's hard to be the only person expected to sit still and be quiet with no explanation as to why.

But once you know the range of social settings where you can expect your kids to behave, you can opt out of certain situations. I have two kids, both are now college age. We rarely took them to restaurants. It was unpleasant for us, unpleasant for them, and expensive. In fact I live in a town that's known for being a place to raise kids, and most people don't take their kids to restaurants that are not overtly kid friendly.

At the same time, people being more tolerant of the normal range of behaviors exhibited by children, could also be a matter of parents doing their job.