I lived in the same apartment for 23 years in Tokyo and never once had to think about buying a plunger for the toilet. Maybe I was very lucky, but I also don't recall seeing plungers anywhere in office bathrooms that I worked at or visited. Perhaps the plungers were hidden or there was a magic pixie who lived in my septic system that made sure it was smooth sailing all the way, but for the duration of my time there, I never experienced a clogged toilet. Since coming back to the U.S., I've lived in three different places and each and every one has required the services of a plunger at least once. The odd thing is that the "worst" one in terms of frequency of clogs is a Toto toilet, the very same brand that I had in my Tokyo apartment. My guess is the issue is not the toilets, but rather the toilet paper that is sold in the U.S. I bought recycled paper that was cheap and 1-ply. Most of what you get in the U.S. is 2-ply and "quilted". Apparently, the few seconds of contact with your tender parts has to be well-padded here.
I miss the fact that the toilets I experienced, both Western- and Japanese-style in Japan never clogged.