Mikado 帝／御門

Mikado has different meanings:
- the honorable gate.
- he was also emperor of Japan, once the spiritual leader of Shintoism, and revered as a god by his subjects during his time.
But I read that in English, the use of the term Mikado is now totally considered obsolete.
I add this in mind when I made this series, how to mention something that existed without being able to show it.
How something common can become obsolete for contextual interests?