I interpret it as some of the JP booking sites will list the year the property was last refurbished, room listings with mention of refurbishment/"annex" (possibly newer build), and some of the more well known places may not have modernized.

I've also learned to pay attention when booking "famous" ryokans/hotels in "famous" onsen towns like Beppu, Kinosaki, Dogo, Noboribetsu, and Nyuto. I've encountered ryokans resting on laurels and ambivalent service for a ryokan at the level it was known for. It's fine if I've set expectations accordingly and want to enjoy the onsen facilities at almost all hours of day and night but sometimes I'd rather stay at a smaller and modern hotel/ryokan nearby (with a lot less cost, quiet, cozy, & everything clean) and visit with a day pass.