Dietary preferences/vegetarianism

You could always just have your Sims practice whatever dietary lifestyle you want despite not choosing the trait. I tend to make my Sims vegetarian and pescatarian more often than not just by making sure they eat foods that align to that diet.

I remember thinking this while Sims 3 was still in development - it'd be lovely to have these as a separate thing rather than trait slot. But I do as @NationalPokedex suggests and usually just guide them to eat the foods I want them to eat :)