Gluten intolerance is not an allergy, and there is no evidence that its incidence has changed. True allergies are a different story: some have increased, some haven't.

I have a mainstream medical textbook from the 1940s which suggests that the overall incidence of adverse reactions to one food or another is around 40% of the population, but most of those cases are either unknown to the sufferers themselves or to their doctors. So it's neither a new problem nor a newly recognized one. It just got pushed out of sight by medical curricula that devoted more time to new ideas in drug-based medicine and innovative surgery. There are only so many hours in a degree course. For an example of what happens, look at the different editions of Bryony Thomas's textbook on dietetics (the standard one used by professionals in the UK). The early editions had a large proportion of the text devoted to special dietary requirements to suit metabolic idiosyncrasies, including things like coeliac disease and G6PD deficiency. The current one has none of that. If you don't have an absolutely normal metabolism and immune system, a UK dietitian will have had nothing in their training to help you.