...that her tastes have changed, that she's grown out of it, or is it possible that she's feeling a little off, due to the heartworms?

I've not been around enough dogs with them, at any level, to know if that is likely to be the case, or not.

At least her snacks have been healthy. Tango, who didn't get much until old age, either, also liked fruit and popcorn. And, vegetables.