@kurolegacy if you mean this specific author, I think this is the first time Neptune has shown up in his doujinshi at all, but I also notice Neptune is not really a popular character on her own unless the artist is specifically lewding her with Uranus or doing a gimmick like this.
I'm actually surprised NTR isn't more popular with her, given the recent anime re-do has Uranus flirting it up with Sailor Moon so aggressively.