The Dead or Alive series is an interesting concoction. It's a 3D fighting game, which isn't terribly unusual, but many of its contestants are what you'd call scantily-clad and amply-proportioned. That's emphasized by the existence of the Dead or Alive Xreme Beach Volleyball spin-offs, which shift the focus from fighting to, well, other things.
Despite the obvious appeal to sexuality, developer Team Ninja has been outspoken in the past about fans going too far with that sort of thing. In 2015, just ahead of the release of Dead or Alive 5: Last Round—the first DOA game to come to PC—game director Yohei Shimbori asked PC gamers to "play our game in good moral and manner," and warned that the studio wouldn't release more games in the series on PC in the future if things got out of hand. He didn't specifically cite nude mods, but the implication was clear.
(In case there was any doubt, by the way, it took about 24 hours for the nude mods to appear.)
Fast-forward a half decade, and it seems that little has changed: Publisher Koei Tecmo announced today that it is bringing legal action against the seller of a DVD that contains nude-modded footage from games including Dead or Alive Xtreme Venus Vacation. The original statement is in Japanese, but Siliconera said that despite the modifications to the footage, the publisher considers it a case of copyright infringement, and aims to file both criminal and civil charges as a result. That fits with a Google translation of Koei Tecmo's announcement, which also warns that "we will continue to take strict measures against copyright infringement."
Interestingly, while Dead or Alive 5 and 6 are available on Steam in North America, Xtreme Venus Vacation, a game in which players invite women to a private island to take part in competitions to determine the strongest and most beautiful of all, is not: It is on Steam, but only available in Japan. Maybe we shouldn't have made those mods after all.
Update: Koei Tecmo has released an English-language statement confirming the legal action.
Reproduction (including some modifications), distribution, and distribution of our copyrighted work without our permission is an act that infringes the rights of the copyright holder and is illegal.
We will take decisive measures against such illegal acts. We will continue to take strict measures against copyright infringement and strive to provide games that satisfy our fans.
We don't deny derivative work but we may take action based on
a) discomfort felt by our fans and
b) the feelings of the developers.