Chefs were once told never to cut lettuce with a metal knife because it was thought the leaves would brown more quickly. People were told to either tear the lettuce or use a plastic knife to preserve it longre.
We now know that's not true.
A Cook's Illustrated report on plastic lettuce knives found lettuce cut with a metal knife showed very faint browning on the edge after 12 days, while lettuce cut with a plastic knife browned slightly after 13 days, and hand-torn lettuce lasted 14 days. And who keeps salad around for two weeks?
What is probably responsible is there are now more sources of information. We don't have to depend on word of mouth. A survey of books for both professional and home cooks that date back to the mid-1990s show even then, all were already dismissing the old rule on not cutting lettuce.