This isn't quite correct. What was - and still is - made available to customers willing to pay for it were listings, generated by the compilers, not the original sources. There are more bits and pieces besides LMF that are excluded, sometimes for security reasons, sometimes for legal reasons. Also not made available, are any of the build procedures. What was never done, is release the sources under an open-source license. The source listings are intended as a debugging aid, not as a way to rebuild the OS.