What can I use for a flat on/off light switch that won't yellow?

@Phyl, you didn't say at what point in the last fifteen years you noticed the flat buttons had yellowed. If somebody in your house smokes, the nicotine will definitely yellow some plastics, plus any fabric/textile, plus the windows and much more. If it were only one switch, I would guess sunlight, but it's probably not that. The plastic just might not be made to stay white for fifteen years--shame on the manufacturer. Anyway, you can buy replacement flat switches at the big box home improvement stores and in lighting and hardware stores. Be sure to turn off the electricity if you are going to work with electricity. OR, you could use a half inch flat brush and paint them white again. Maybe the switches will last for another fifteen years. Best wishes 😇

Use contact paper, wall paper or colored tape to cover what you have. Each is removable if it is not to your taste.

try nail polish remover, a guy that works at home depot gave me this hint about 5 years ago when helping my son flip a house that had so much smokers damage that the ceilings needed 5 coats of kilz to cover

Paint orcover with contact paper

Switchplate covers are relatively inexpensive. Take the old ones to the store with you to purchase new ones.