I am unsure if this is a duplicate idea, but if it is, that is okay, at least it is already being considered.
There should be a Spotify feature where you post a rare or hard-to-find album by an artist on Spotify, or other albums not normally found on Spotify, and not get in trouble, in an isolated publishing area. This is where the original artist will still make money from whatever is posted regardless and not the listener who posts the copyrighted material, who must understand that they aren't to make money for what they post and their real intention is for the actual artist to make money from something they should put on Spotify in general, and to make it accessible to others.
The unofficial posts should be tagged or labeled as an unofficial post, and there should be an option to show or hide the unofficial posts on the artist page. Spotify must also check the quality of whatever is posted just so A) they can correct the metadata information to associate the post to the right artist, B) determine and assure whether or not the sound quality is good for people to consume the music, and C) check for duplicates.
As far as the artists go, in order to post an unofficial release, the artist must already be a member on Spotify, for this is how the artists make money from people playing the music. That means if Def Leppard is nowhere to be found in the database (by the way, their live stuff is), an unofficial post that has to do with Def Leppard will be rejected.
Hi, thanks for your suggestion! Unfortunately, I'm going to mark this 'Not Implementable': Spotify would be in violation of their licensing agreements (and copyright law in most jurisdictions) if they allowed unofficial releases to be contributed by individuals others than the rightsholders.