https://www.elliott.org/on-travel/should-you-recline-your-airline-seat-no-and-heres-why/
Should you recline your airline seat? The passenger sitting in front of me on a recent flight never bothered asking. Shortly after takeoff, he just lurched backward. No warning. How rude.
“Unless you were sitting in a seat with extra legroom, or in first class, it would be inconsiderate to recline your seat,” says Diane Gottsman, an etiquette expert who runs the Protocol School of Texas. “Space is tight, and it’s common knowledge and no surprise that you will be sitting in tight quarters.”
It used to be fine, back when there was enough space (34 inches between economy class seats should be a standard). But those days are long gone. In economy class, I used to have enough room to cross my legs; now I can’t even move my legs after I’ve taken a seat.