The answer is, long ago is when they stopped making it walkable. Much cheaper and lighter to just have a roof than a walking platform
You can get a 2x3 foot piece of plywood (approx size) and lay on the roof and crawl on all 4s and get p there. Hopefully the wood will spread the load to the roof joist so you do not bend it too much. Being up there will flex the roof and the places the roof is glued or somehow joined to the next roof section may become weakened by all the flexing and cause a leak. A friend's teenage boy would walk on the roof (and he weighted abut 110 lbs) and eventually the roof leaked.
Do as much work as u can from a ladder, and be on the roof as little as possible and spread out the weight as far as possible.