1/4" solid walnut, or a walnut veneer over an engineered wood product of some sort?
Walnut should, of course, be safe. Any treatments to the walnut, whether during manufacturing or applied afterwards (did previous owners polish it regularly?) are unknowns. If it is not solid walnut, what exactly was the formula used for the engineered product? I would expect any smoke residue to be just that, surface residue that could be cleaned off. It is my understanding tobacco smoke is something people who restore paintings deal with a lot. They would presumably have methods of cleaning that walnut could stand up to if you want it cleaned and don’t want to go with the “that should work method.”
As far as hazardous material released by the glowforge, the thing to consider is how much of it will be vaporized? The depth of material removed is 0.25 inches and the width will be roughly .008 inches. The length of the cut(s) will get you volume. If you’re engraving, add that volume in. Most of that removed material will be particle smoke, but some will be other compounds either freed into the air or products of combustion. So the question is what will the concentration be and what concentration is 1) corrosive or otherwise harmful to the glowforge, 2) harmful to health considering it will be vented into a large volume of air and presumably breathed mainly by squirrels
(unless you vent over a sidewalk) and 3) harmful to health residue buildup inside the machine encountered while cleaning (hint use gloves.)
My main concern would be if it is truly solid wood and what was used to treat it or clean it. Personally, I wouldn’t be as concerned about what happens to tobacco smoke when it is burned again. Concerned about what walnut that hasn’t been cleaned of smoke residue would feel like and smell like, oh yeah, that definitely.