In this answer, Liam W said [emphasis added]
'A Japanese' infers the Japanese person is a thing, and not a person. This is what deems it offensive.
'A Japanese Person' infers the Japanese person is just that - a person, and is therefore considered fine for use.
While not wanting to go this far in using simple words, should we try to use simple and common words that are easily understood by people learning English?
An exception to this would be technical terms to do with English, such as "split infinitive". They should still be used, because it is easier to google for "split infinitive" than "boldly putting a word between "to" and the verb".