I don't frequent fine-dining establishments a lot. Good restaurants, yes, but not Alinea-level restaurants. When I do, I'm putting myself in the chef's hands, save mentioning that I have celiac disease.
When @cyalexa and I dined at the Catbird Seat in Nashville, we had no idea what each of the courses in the 10-course tasting menu was going to be, nor what it was going to have in it, until they brought it to us. (They gave us a copy of the menu to take with us afterward, so we could remember what we had.) I would not have dreamed of asking them to change the prep of anything to fit my tastes, even had I known what it was going to be.
They did ask at the time we made reservations whether we had any allergies, etc., that needed to be taken into account. As that was before I was diagnosed, we did not. It was, btw, a stunningly marvelous meal.
I have no problem asking the local steakhouse to leave the pepper-heavy seasoning off my steak. They seem to have no problem doing so.