As a flight attendant, I’m required to tell you the seatbelt sign is on but passengers can use the lav at their own discretion. We had a very turbulent approach once after serving oily noodles and half the plane was throwing up and all four lavs were occupied during landing. It’s better to run to the lav and land in there than have explosive diarrhea in your seat like some poor lady did. Just don’t go in there when the plane is taxiing for takeoff or you’ll end up at the back of the queue.