Standard data folder.

Have you tried deleting it in the mod manager?

How is that not stupid?

And besides the mod manager, does the mod manifest itself in any other way? Does it actually work in the game?

Yes, it works in the game. The first time, I didn't think to check the UserMODs folder using the mod manager. I reinstalled Heroes, but it didn't help, so I had to reinstall the OS. Now, I used the mod manager to delete this mod, and the effect disappeared.