We need a more nuanced definition. It's not precise enough to say:
If it isn't unique to vegetarians, it's not really necessary to ask here.
That sets an extremely high bar that excludes a large set of interesting and useful questions. There's really very little that is truly unique to vegetarians, especially among food. Tofu is not the exclusive property of vegans. People who are lactose-intolerant are interested in dairy substitutes, even if they still eat meat. Any food that a vegetarian might eat is probably also eaten by a non-vegetarian somewhere. So are we going to blacklist questions about food on a site that is primarily about food? I don't think so.
The thing that makes vegetarians and vegans unique is that they live within some generally agreed-upon constraints which are not followed by society at large. So I propose a different test.
Questions must relate to constraints that vegetarians live within and the challenges that arise from those constraints.
This is similar to the idea that Erica and others were trying to express, but the wording is more flexible. Now a question about meat substitutes would be on-topic because it relates to a vegetarian constraint against eating meat, even though tofu and the Beyond Burger are eaten by plenty of people who aren't vegetarians.
But what counts as a vegetarian constraint? Not eating meat is an obvious one. So too for eggs, dairy, and leather, even though those restrictions are followed by a smaller group of vegetarians (the vegans). But those aren't the only groups we have here! We also have fruitarians and raw foodists who choose to avoid most vegetables and cooked foods, respectively. They might not be popular lifestyles, but the restrictions are valid within their framework. It's up to the community to determine which lifestyles we accept here -- I suspect that we won't be allowing questions about breatharian living any time soon. Meta is the right place to make these decisions.
Let's see how this plays out with some of the recently contested questions.
How to remove pesticides and harmful bacteria from fruits and vegetables? (borderline)
One of the useful functions of cooking food is that it kills bacteria. This person's problem could be avoided by cooking their food, except they have adopted a constraint against cooking food. That constraint is valid within their chosen framework. This is why it's very important for questions to be tagged with the appropriate lifestyle or diet pattern, so that we can answer the question with respect to the chosen constraints. In that regard, the question is on-topic. However, the bit about pesticides does seem to off-topic because it doesn't relate back to any vegetarian constraint. This question is borderline and could benefit from editing (the title especially) or clearer tagging.
Are mango seeds edible? (fail)
I don't see how this relates to any vegetarian constraint. By this test, the question would be considered off-topic. I say that while being fully aware that I have voted to reopen the question and it currently has 4/5 reopen votes. I voted to reopen based on precedent, not definition.
Is boiling orange or lemon peels and drinking it as a tea healthy? (fail)
There's nothing in the question that relates back to a vegetarian constraint so I can definitely see why it was closed off-topic. It was nearly redeemed by the answer which shows that orange and lemon peel are a good source of pectin which is a vegetarian substitute for gelatin, but that would really work better as a separate question. Conclusion: off-topic.
What place do chia seeds have in cooking? (pass)
Chia seeds are an excellent source of omega-3 fats. This relates directly to the constraint against eating fish because oily fish are (currently) the world's largest source of omega-3 fats and therefore it might be a challenge for vegans.
How can components of a packaged food item exceed the net weight? (borderline)
If the question had been asked in the context of a low-carb diet (eg. vegan keto) then it could be on-topic based on the constraint to reduce carbohydrate intake. However, the question wasn't asked in that context, so it doesn't seem to relate to any vegetarian constraint. It also meets the bar of being substantial concern for non-vegetarians because understanding nutrition facts labels applies to nearly everyone. Migrating to Seasoned Advice would have been the right choice.
Okay, that seems like it works fairly well. And from a quick glance at the most highly upvoted questions on the site, I'd say that our best questions all fit this test as well.
And just to round out the definition, I'd like to add a supporting corollary.
Before migrating a question it should be shown to be a substantial concern for non-vegetarians.
This sets a pretty high bar for migrating questions to other sites. Most people will live out their entire lives without ever boiling orange or lemon peels. That's not a substantial concern, so we shouldn't migrate it. If we don't like the question, we can just close it. Likewise eating mango seeds is not a substantial concern, so it shouldn't be migrated. However, arsenic in rice is a substantial concern, and could merit migration. Even though vegetarians eat a lot of rice (with beans!) it's also important and common for a lot of people who don't restrict their diet.