yeh its an 'undocumented' api, but microsoft are usually backwardly compatible with their setups, and some of their code also uses these api's, so if they aren't really going to break their own programs
..
look at the peb / teb blocks from 9x->xp, even on x64 platform, they are backwardly compatible, and only have new stuff added with each os build..
its like their documentation, often its quite wrong on some api's / structs / params... microsoft = do as we say... not as we do