[codedokode]

The information about many internal "undocumented" structures is actually documented on third-party sites. And if there are structures that are really not documented anywhere then you don't have to reimplement them because no developer is using them and they are not necessary for compatibility.

[intea]

So you're using their "leaked" or reverse engineered information? That's just IP theft with extra steps.

[codedokode]

If this is made to ensure compatibility with programs using those undocumented structures then it is a different thing. They become a part of API.