I have a TDS540, that has not been powered up in a couple of years (maybe more!).
When I look from the side, in the right light, I can see some crazing or some wrinkling on at least the top half of the screen (looks like the surface of the plastic, in front of the crt screen, has been damaged by cleaning with a solvent; but, its not the plastic... it's below the plastic.).
Given what I am seeing, I am assuming it is the gel that has degraded?