Whether there is food at all
On short flights within a region, assume there is not — or that there is a trolley selling sandwiches. On long-haul, assume there is, because the flight is long enough that not feeding people becomes a welfare question rather than a commercial one.
Between those two the answer is the fare. Airlines that unbundled everything sell food on routes where a legacy carrier still includes it, and the same airline can do both depending on which brand the ticket was sold under.
The reliable move on any flight under four hours is to eat before you board. Everything below is about the flights where that is not practical.
Special meals are a separate booking
A vegetarian, kosher, halal, gluten-free or low-sodium meal is requested against the booking, usually up to about 24 hours before departure, and it is not part of choosing a seat or checking in. Miss the window and the galley carries what it was loaded with.
They also arrive first. Special meals are served before the main service because they are counted and tracked individually, which is a small and genuine advantage if you are in a hurry to sleep.
The failure mode worth knowing: a special meal ordered on the outbound does not always survive a schedule change or a re-issued ticket onto a different flight. It is worth re-checking after any change to an itinerary.
Why airline food tastes the way it does
Cabin air is dry and the cabin is pressurised to something like 2,000 metres of altitude. Both dull the perception of salt and sweetness — by a substantial margin in most measurements — while leaving sour, bitter and umami closer to normal.
That is why the food is seasoned harder than it would be on the ground, why tomato juice is drunk on aircraft by people who never order it anywhere else, and why the wine that tasted fine at altitude is disappointing at home.
Timing, and the thing nobody tells you
Service starts after the seatbelt sign goes off and takes a while to reach the back. On an overnight flight that is the difference between sleeping and eating, and the choice is worth making before you board rather than when the trolley arrives.
If sleep is the priority, tell the crew on boarding and most will leave you a tray or skip you entirely rather than wake you1.
Some airlines offer this formally as an express or sleep service; on the rest it is a request to the crew, and it depends on how the galley is running. ↩