Yes, it's very much anathema to put ketchup on a hot dog, at least among Chicago hot dog enthusiasts.

For those unfamiliar, Chicago is also one of those American cities with its own style hotdog, so it's something of the local culture:

> All-beef frankfurter, on a steamed poppy seed bun, topped with yellow mustard, chopped white onions, bright green sweet pickle relish, a dill pickle spear, tomato slices, and a dash of celery salt.

From https://en.wikipedia.org/wiki/Chicago-style_hot_dog

Obviously, this is only as serious as you take hot dogs, but they are very good and compared to deep-dish pizza, the Chicago-style hot dog feels almost healthy.

Yes. I think at this point it's more of a meme than a trend, but tbh I will not take Ketchup on my Hot Dogs / Red Hots just as a shibboleth signal.

That said, there are exceptions (my sister is dead to me though...)

they certainly don't appreciate it on their hot dogs.

being from the non-Chicago part of Illinois, I love piling ketchup on hotdogs in Chicago just to see the looks of disbelief and scorn. Makes the hotdog taste that much better!

Beyond the age of 8, apparently.