Hi! Cool question. I would say, in my experience as a ler, the lee either starts the session barefoot (quite common) or the lee has shoes/socks on and as the ler, you take them off. I’ve not seen any videos where socks come off by accident.
In real life, I had one experience with a female lee that was being tickled on her feet and she was frantically trying to get away. I had a nice arm lock on both ankles and she had on white ankle socks. She was so frantic kicking while I was tickling that both of her socks came off as she was pulling her feet out of the arm mock I had on her. She pulled both feet out of my arm lock and her socks slid off.
After a few minutes I strapped her arms/legs down on a table and continued to tickle her feet (without those socks) without any more resistance from her.