In an ideal World, this is something that should be easy to fix - a list of ingredients. However, the crews and the airline won't know either. The catering is delivered in bulk by a third party catering company, in frozen boxes, which are put into the ovens, then onto dishes.
Crew food does have a (very long) list of ingredients, that we can read, since crew food is given to us in its original packaging - no posh dishes for us !
However, probably nobody has thought of this - would you expect any better response if you asked the same question in a pub or a cafe serving food? Air crews at airlines I flew for do have a detailed pre-flight briefing, and are made aware of those with peanut allergies for example, since that presents a medical risk, (and the cabin crew are extensively trained in first aid, including anaphylactic shock and use of epi-pens and defibrillators).
It is sadly, not an ideal World, and having alternatives in today's very competitive environment is not going to happen - beyond perhaps "chicken or beef?". So as others have said, those with allergies are well advised to take their own precautions, (and a doctor's letter if necessary).
Am a bit alarmed to notice sodium ferrocyanide in that crew "meal" pictured above, which I subsequently ate. Isn't cyanide a poison ??