Not only are we more aware now of certain allergies and allergy related conditions we're still learning.
I became vegetarian for ethical reasons 30 years ago.
All my childhood I'd had a "dodgy tummy" and was prone to heartburn, diarrhoea, trapped wind, very long lasting and painful hiccups.
After turning veggie much of those symptoms vastly improved if not disappeared altogether.
However, at the time vegetarianism wasn't as prevalent or well catered for as now and certainly in restaurants/cafes it was not uncommon for supposedly veggie food not to be.
I got caught out a few times with dishes containing gelatine or meat stock, I came to this discovery after a few times of it happening and each time I'd ended up with a really awful stomach upset. Fish and chicken weren't a problem but red meat most definitely was!
I've noticed it since when I've been lax about checking products when they've done the "new recipe" nonsense.
About 7/8 years after I noted this issue personally it was in the news about red meat being bad for the bowel generally. My family and I weren't at all surprised.
From discussions with other veggies and gastro specialists I genuinely believe a fair number of people are allergic/intolerant to red meat.
After it was in the news about the link to bowel cancer etc my parents also quit red meat - my dad had very similar issues to myself and again found things improved a lot when he stopped eating red meat.
My brother and sister aren't similarly affected but my sister is allergic to fish so can understand from that perspective (though for her it's full blown hives, sweating etc with the slightest contact! She definitely wouldn't have lived in days gone by).
Similar to pps I never liked meat anyway I found it really hard to chew and pretty tasteless, but I was designated "fussy".
When I went veggie and suddenly turned into someone who happily cleared their plate and would have seconds and generally was eating more and enjoying eating my mum voiced regret at not letting me do it when I was younger as I wanted to. I'd always been a very slim person and "sickly" (combination of other conditions inc asthma, eczema, hayfever, frequent tonsillitis, then later endo and associated anaemia, migraines etc)
One oddity was my mum insisted I keep an eye on iron levels and what actually happened was they improved! I've not been anaemic once since turning veggie but had a few bouts prior to turning veggie.
What makes me most angry though is that my parents had 2 kids in the house with asthma and yet continued to smoke heavily in the house and even worse in the car!