Everything has to be evaluated on a one by one basis, it would be impossible to say deck structure damage doesn't matter with a keel stepped mast, it's not as important but you normally have to have some structure up there. Most keel step masts I've seen do use deck support. The amount of weakness due to lack of core is very difficult to estimate without actually seeing and feeling it in action. I would say though that you could easily support the mast at the deck height with
lines or cables and have little worry about sailing due to mast deck support failure. If you were careful to not get into a stressful situation, reducing sail in high wind, you could probably get by without doing anything to move the boat to a new location to do the deck work.