OP, it's very easy. The host is under no obligation to provide a choice of food at an event. Their guests are equally free to decline an invitation. Why would I go to an event where I find the food unpleasant and I'm going to be left hungry?
If I hold an event, I offer a choice of starter - fish or veggie, and a choice of main - meat or veggie. Dessert is generally fruit with optional cream, and a dairy-free sweet cake or pastry. I let people know the menu in advance so they can decide if it is for them or not.
At a recent work event, a new member of the events team decided to only offer veggie/vegan on the basis of ethical values and her complete confidence that everyone would like her veggie food as much as she did.
Unfortunately the 60 strong engineering team took one look at the buffet and then left en masse for a nearby steak/burger restaurant. 😂 You cannot ask people to travel 200 miles and not feed them. We had to bin 90% of the buffet.
Equally I attended a conference at a university last year that had gone completely vegan. I will generally eat anything but this was inedible.(higher ed budgets?) We were slightly more tactful, ducking out in shifts to get food elsewhere but I haven't rebooked that event. The sales team would refuse to go.
As for 1/3 vegan, 1/3 veggie, 1/3 meat, this is way off in my experience. For London-based consultant lunches (ages 30-60), we normally get one vegan and one veggie to eight meat eaters.