Looking at the rules it seems that it's not allowed, but most of the version of the game you find now (gog mostly) is running it.

What would it change to allow it?

Should'nt we create a new category allowing grayface patch?

The GOG release does not include Grayface's patch, only Mok's patch, which is why Mok's patch is also allowed on the retail version.

There is no need to allow any other third party modifications.