Ya idk what Japan's law is on modding games but if it's on your computer you have a right to mod it and if you're distributing it for free and not packaging it with unlicensed intellectual property then there's nothing to sue over.
Absolute worst case scenario would be the fan translation getting cease and desisted, and since Shade is distributing for free even that's unlikely.
Not saying we should rub it in Inusuku's face. Since lewd content for this game isn't on the unpatched Steam version I don't think anyone should be contributing to distributing this on the Steam platform. Let the steam players find it themselves, but it's super duper unlikely that there'd be any legal ramifications to anyone for it.
The legal ramifications aren't for us; rather it's for the mod developers, or this website.
They cannot target a nebulous mass. They can, however cause a C&D on these mods and perhaps the website, especially if the aforesaid user-mod developers and the website are in America.
Even if it's unenforceable, there is still the fact that Inusuku dislikes having their things modded. They might quit developing the game entirely, because Japan is rather weird with how frequently they nuke their artist accounts.
I can at least understand it from an Artist POV: mods that can vastly improve the game may make the author feel like their initial product is inferior or they no longer have control over it.
Really, best to err on the side of caution. If people want the mods, they'll eventually get here themselves, one way or another.