We use the old Kukki TKD belt system, but we haven't taught a strictly TKD curriculum in decades. Over the summer we were shut down do to COVID but our school has a courtyard behind the back parking area so we were able to run classes outside. We ran classes for about 3-4 months wearing normal clothes with no belts or rank and honestly it was some of the best classes we had. If so many people didn't put so much stock in belt rank we would probably just get rid of them. It's actually very common for us to just wear normal gym clothes in our black belt classes.