is JP. The other has been considered for a long time as a slur because it was so often used as such that it became so to an entire generation. There's no quick way out of these situations. Use "JP" and you're ok.
I was making a post and trying to reference the women's soccer match between the USA and Japan, "US vs J@p women's soccer match" but it seems that the initial used for Japan is censored? I would have just edited my post and written the full name, but it seems you can't edit post? So anybody reading my comment isn't going to know what country I was referring to now.