> ... (although I think UTF-8 should not be used for Japanese either) ...

The people putting up websites in Japanese disagree with you, it would seem. According to Wikipedia (in the Shift JIS article), as of March 2026 99% of websites in the .jp domain were in UTF-8, with only 1% being in Shift JIS.

Japan used to have two different encodings in common use, Shift JIS (usually used on Windows) and EUC-JP (more common on Unix servers). This resulted in characters being misinterpreted often enough that they coined the word mojibake to describe the phenomenon of text coming out completely garbled. These days, it seems Japanese website makers are more than happy to accept a slight inefficiency in encoding size, because what they gain from that is never having to see mojibake again.