For the kinds of discrimination we are talking about, I would never expect a written policy to emerge as evidence. Written policies would be scrutinised by legal departments; any written policy that unmistakeable referred to clearly prohibited activity (like passing over a particular race) would never be sanctioned.
Interesting.
So you would say that if someone wishes to investigate the realities of institutional racism, they would first need to think critically about what forms of evidence are and are not likely, and the many avenues other than clearly stated law or policy by which institutional racism might occur?
You might find the published works of former Harvard Law professor Derrick Bell quite interesting, as he dedicated his life and writings to assessing American law through just such a rubric of critical race theory.