I think it's inaccurate to apply that term to a nation that is (mostly) the exact same as a single ethnic group on their traditional lands. Barring the colonized Hokkaido and Ryukyu, the Japanese people formed a unitary nation state on their own land out of their own historical conditions. They're fairly ethnically homogenous not because they're an ethno-state in the modern sense but because Japan is where Japanese people live (big brain ideas here).

is it the prefecture with 80+ consecutive years of population decline?

etho-state or ethno-state?