Gluten allergies are relatively rare and extreme. People with Celiac have a major, sometimes life threatening reaction to gluten.

People who do not have Celiac and say they are gluten sensitive usually are not. They are either experiencing a nocebo effect or have gas from other components of wheat rather than gluten. Actual non Celiac reactions to gluten are minor if they exist at all. So when someone says they are gluten sensitive it is overwhelmingly likely they either have Celiac and even a very very tiny amount of gluten will cause them horrible consequences, or they are incorrect and they are not sensitive to gluten at all.

https://pubmed.ncbi.nlm.nih.gov/27523634/

https://pubmed.ncbi.nlm.nih.gov/29102613/

That day could be today: https://www.molinograssi.it/en/retail-line/miracolo-line/

From what I understand, most "ancient" grains had far less gluten; it's the modern variants that have caused the widespread gluten issues we see today.

That's obviously an oversimplification.