There's a bunch of mods for closed-source games on Codeberg such as Minecraft mods. As long as the mod itself is free and open-source, then there seems to be no problem.

Tldr: don't let Perfect be the enemy of good

I don't know about Codebergs policy on the matter, but morally I think there's nothing wrong with putting open-source mods for closed-source games on Codeberg.
I always use FOSS software whenever possible, even if they're lacking in some aspects compared to closed-source alternatives, but have no problems with closed source games. Games are entertainment, not utility. Games have a short lifecycle compared to utility software. Games are often a one-time experience, and when you've finished a game it's done. (Nobody ever "finished" their use of Notepad). Meanwhile developers gotta eat.

There's also some precedence for open-source projects that can only be fully accessed with closed source software, like open-hardware using Eagle for PCB and schematic design (before KiCAD truly took off), or Fusion360 for CAD ( FreeCAD development is accelerating though)

Thanks everyone. I'm moving to Codeberg.

Codeberg has a matrix room where you could probably get an official answer to this if you need, but iirc they recently clarified what content they prefer to host and I don’t remember anything like this being excluded

If the mod code itself is open, i dont see why not. There are lots of open source projects that are used in conjunction with closed source stuff, like custom wrappers or security research tools.

I think you're best asking a lawyer, to be sure.

But from what I, random citizen, have looked into the matter out of curiosity, apparently they're a grey zone, usually overlooked, ignored or accepted. But while you'd be sharing differential code, not the protected code itself, you needed to break a patent by reverse-engineering the game, which for draconian laws like the DMCA, is potentially worth even federal prison. Plus for others to apply the mod, they'd need to break the patent too by using the tools you indicate.

And there are cases where the game has official modding tools or that the devs explicitly say they're fine with mods. In such cases, I'm fairly sure worrying isn't needed.