Back in the late '90s they used a new flag (black and orange quartered?) for a year or two but it dropped out of favour for some reason. I think competitors had to slow down to a 'safe' speed (maybe 50 mph) and not overtake but I don't think you had to hold station, so in the end a bit like a safety car situation.
I did a race at Mallory and a car went off at the esses and had to be moved so this flag came out and it was all a bit chaotic, next time round the lead car went into the pits, drove slowly straight through and came back out in about 4th place. The three of us who went past (in accordance with the regulations) were all called to the clerk of the course after the race to explain why we had passed the leader. Race control hadn't noticed his pit road detour!
Another competitor slowed down to about 30 mph and we passed him but no mention made of that.