In this case, I would just ask the owner to elaborate as to what they mean by “table scraps”. Though, a common thing a lot of my co-volunteers and I do is give green beans, yogurt (this can be great for food transitioning or oral medications), or small bits of unseasoned lunch meat with meals or for use as treats. As far as safety goes, my general rule of thumb with people food is this: If you are unsure whether or not it is safe, do not give it.
Another thing to keep in mind is that some dogs may have allergies or special conditions, so you will want to check with the pet owner and ask this if they make this type of request. For example, my cat is allergic to most seafood (salmon especially). Something like this can be easily forgotten by a rushed/stressed pet owner, so it can be crucial for sitters to ask.
I, personally, will withhold treats, people food, or table scraps if I feel a pet is unhealthily overweight/obese.
Like with people, being overweight can cause several issues with health / the body, and I’ve seen first-hand how excess weight can affect dogs and cats.
If I notice a weight problem and the owner makes a request for extra “special” foods, I will respectively inform them that I am not comfortable with the situation.
I hope this helps. A lot of other comments have some friendly tips too, but I would definitely do your own research so you’re in your comfort zone! Good luck!