Okunoin (奥の院) is the world's oldest still-used graveyard. It's located in a cedar forest on a mountain and nature is constantly reclaiming the monuments.
Beautiful. But is it "still used?" I went down a rabbit hole and the only mentions of use other than visiting is people wishing to scatter ashes or place a lock of hair on the grounds.
Well there are a good array of modern corporate memorials like Panasonic. Some of the corporate memorials have pictures of former employees that are added to over time as well.
There are other very shiny newer monuments in parts too but i don't know how old they are. The main path is 2km long, it covers a lot of ground.
I just found Johnny Kitagawa's grave is there. He died 2019, and his ashes were sent to various celebrities but here's the grave. Google translate gives enough info
There's a few things going on with the corporate ones. Companies started building them in 1927, starting with the Asahi Shimbum (newspaper) who put up a monument dedicated to their deceased employees. Often they commemorate employees who have passed like the Nissan cenograph . Some of them have individual photos of the employees, usually from around the 40's onwards. Previously in Japan the whole idea of the place you work being "family" (and you working the same job your entire life) was a big thing to foster loyalty. Much like the "we're a family here" bullshit line you hear in the West, but the company will at least name you on a piece of granite when they overwork you to death.
Some of them just commemorate the founding of the company, and tend towards a status symbol humble-brag and show off how pious they are at the same time. Sort of like how In-n-out puts bible verse numbers on their cups and Chik-fil-a doesn't open on Sundays. I'd say the Denka one is along those lines. It was built for the company's 100th anniversary.
Some of them are used as gathering points for annual memorial ceremonies too like the Yakult one complete with iconic bottle design, I know there's a tradition in at least some parts of Japan to return yearly on the anniversary of a funeral to visit the family, many of these stones serve as gathering points for the companies to do memorials. Some graves are for individual business-people who were important to the company like presidents. The Panasonic grave was put in the day after they changed the name to Panasonic, the block on the left of it in the image is their original company name gravestone. They were super keen on keeping it updated.