My daughter had a yeast allergy as a toddler (along with several other allergies) but now she can just has to worry about yeast buildup so she can eat it sparingly.
At table service restaurants and buffets she ate chicken or steak with potatoes and steamed veggies, we always had to speak to a chef to prepare her meals. At quick service, best to read the allergy binders as yeast can be in so many things and the ingredients change often at WDW.
You can occassionally find the allergy chicken tenders without yeast but there are a few brands that they use so you have to look at the ingredients at each location. Popcorn and the roasted cinnamon nuts sold at carts were free from yeast the last time I checked. You can find fruit around the parks like watermelon and bananas. Allergy waffles at resort quick services and table service restaurants may also be yeast friendly- there are a few different brands that are used, you can also pair that with freshly made eggs (be careful of the scrambled egg mix they serve at QS as it has a ton of ingredients). Plain hamburger patties (no bun) and french fries can be found at many quick service locations.
I highly recommend you bring some of your own snacks, with the limited menus due to covid there's a chance you might not find something suitable for you.
