I'd think tobacco before nicotine, but I'm not an allergist by any stretch.
One that I came up with tonight for my smuggler character (who may actually run into them!) is bovines: cows, bulls, real meat and leather. For a Low lifestyle runner of the city this doesn't really come up except from half a mile away on a field. And a Luxury living runner could just buy friends who don't eat meat or wear leather.

You do mention dairy, but, eh. I don't know if you can actually be allergic to them generally, but hey, future.
The Drugs section in CRB says that drugs like cannabis (marijuana/pot), cocaine and the like are still around, just not as addictive as the new stuff. (They say 'popular', but I think they mean addictive.) So you could be allergic to pot, maybe so mild that you don't smoke it, maybe so severe that you can't drop in on a college party or a reasonably cool concert in a closed environment. Whether it's common probably depends on your surroundings, I guess.
Corn could be a real bad Common allergy to have - they don't mention it in canon much but it's cheaper and easier to manipulate than soy in some ways, so there's no reason it wouldn't play a role in sprawl cuisine.
I'm not sure what they make synthahol out of but it would be interesting to try to live in the shadows while only being able to drink (or kiss people who just drank) real alcohol. i.e. allergic to synthahol.
Whatever they make synth-leather out of, that would suck to be allergic to.
That 'New Car Smell' chemical. Inconvenient for a rigger.
Something they put on circuitry, like flux or industrial glue?
Personally I have a real sensitivity to pineapples (mouth gets a cut-up feeling and feels acidic in a terrible way) and walnuts (itchy), and can barely make out similar feelings from similar fruits/nuts. I'm not sure if tropical fruit or real nuts come into play for most runners, but some probably get them.
You mentioned sulf
ites, the preservatives that come from fruit skins, but what about sulf
ates, the shampoo/conditioner additives? I've been off those for years and have felt less itchy and bad, and I know there's a significant market out there for people like me in that regard.
This is a little complex but an interesting allergy to RP would be the flavoring they use to simulate your favorite food. You'd look like a snob when really you just need the real stuff every now and then (like the alcohol thing above).
Rubber? Are people allergic to that?
There could be an industrial chemical they use to make DNIs/cable/cyberdecks/?? that the PC is allergic to.
Pool cleaner?
Some sort of bathroom cleaning chemical that they use at all the corporate offices?
Your medication!...
I guess the possibilities are endless!