In summary: the question appears to exaggerate the frequency of such questions, these questions are on topic and valuable to quite a lot of people, and the only action worth taking is building on the questions you do like by asking, upvoting, and answering.
These questions do not seem to be outnumbering (or "crowding out") others.
I skimmed through the most recent 50 questions and saw several food safety questions, and only one about a dietary restriction. (Note that the "no butter" sous vide is not about dietary restrictions, but just about whether to include the butter in the sous vide bag.)
I suspect this may be a bit lower than average, but still, only about 1.5% of questions on the site say "vegetarian" or "vegan". I'm sure that when we include all the various dietary restrictions, the overall numbers climb a good bit, and that there are occasional days when we get more than average and the front page shows it, but overall, it seems like a large exaggeration to say that these questions are crowding anything out.
I can find no evidence that such questions have ever been on the scale of half of questions, and there appear to have been many days with no questions in that category.
These questions are just as on topic as anything else. There's not much to say about this.
The question and comments here seem to suggest that these questions are less desirable, and that most people don't want them. I'm sure this is not the intent, but I think this framing is unhelpful, bordering on unwelcoming.
That "99%" is a pretty enormous exaggeration. From quick searches: close to 10% of the US is diabetic, up to 10% is lactose intolerant, over 3% is vegetarian, and there are various food allergies in the 0.1-1% ballpark. It's more difficult to find data on diets, but low-carb diets are fairly common as well.
So, these questions are directly relevant for a substantial number of people, and when we include all the family and friends who cook for those people, the numbers are far larger. On top of that, there's all the times we end up missing an ingredient - it's not just about dietary restrictions!
On a personal note, I have no dietary restrictions myself, but have cooked for people with a huge variety of restrictions, and I've always appreciated the information I've been able to find online. I'm glad that this site is a place for them.
Finally, I'd like to address the underlying idea here, that something might be less desirable because only a minority needs it. This is where things border on unwelcoming. Being in a minority, whether a food allergy or anything else, generally means it's more difficult to find information and support. If anything, this means it's more valuable to handle these topics. And in general, StackExchange is meant to be a place where everyone can participate fully, not just those in some particular majority or intersection of majorities. Let's not go down the path of suggesting that there's a "better" way to cook or eat. That's needlessly exclusionary and divisive.
I certainly agree that questions that aren't about dietary restrictions are interesting, and it's always great to have more of them. It simply shouldn't be at the expense of something else. We gain on general questions by improving in that area, not by maligning or targeting other areas.
Of course, this is basically the same task as site growth in general, and there are no easy answers. The most direct things any given person can do are to ask more questions themselves, to upvote others' questions, and to strive to answer them well. As always, this is something I very much encourage, but of course, no one person (not even moderators) has the power to change it single-handedly.