Thanks for your response. I agree that certain groups (with certain beliefs) are most guilty of perpetuating this myth, but find it’s far from limited to Keto / low carb. It seems to be mainstream, and include some — but not all — herbivores (based on my anecdotal experiences!).
Protein combining comes up often in my conversations (real world and online) with vegetarians and vegans. Indeed, I published a similar article in a Canadian magazine (Impact Magazine), which I can only assume meant that the editors felt it was relevant.
I appreciate your highlighting the origins of the myth and have edited my intro accordingly (also keeping the myth’s scope more vague!).