This seems extremely unlikely, given the relatively small percentage of beef that comes from culled dairy cows, the absolutely vast amount of beef used as an input for fast food, and the named suppliers companies like McDonalds use.
With chickens it's even worse: males are killed immediately after hatching, because their meat is worth next to nothing. Females are kept for both eggs and meat.
Poultry farmers raise both sexes and slaughter as soon as the bird is large enough.
You can also go to specific chains and look up their sourcing; Wendys sources from "steers and heifers", but all much younger than the average dairy cow cull age.
It's not that I have a high opinion of fast food beef. I just think it's unlikely that they could come close to meeting demand by sourcing from culled cows.