This, afaik, does not exist. Nor do I think it will, having been party to discussions many times about the benefits, with the conclusion being drawn every time, its just so open to abuse.

However a site was conceived a while back where unpublished artists would post unmixed multitracks, and it still exists afaik, having been referred there about 3 months back. I'll check if I logged the site address but if its raw practice material you are after, definitely a few places exist for sharing this.