We were talking about something like this a few weeks ago. The vast majority of samples on major label releases - be it rap, pop, country, whatever - is cleared. No issues there. But most underground rap shyt has uncleared samples. The $$$ generated from underground rap isn't enough for most artists to pursue lawsuits, because you'll end up spending more on legal representation than whatever $ you'd get from uncleared samples on a Roc Marci or Curren$y album.
But imagine Spotify, now that it has this partnership, reaching out to sampled artists and offering to pay legal in order to pursue legal action and ultimately remove underground rap from the platform. Maybe they offer higher stream payments to the original artists - many of whom make obscure or unpopular music anyway - in order to help them collect their legal cash.
Bandcamp might end up being the last place you can buy underground rap besides the artist's official websites. But even that site may not be safe in the long term, given I've seen people have some pretty ugly complaints about new ownership/management there.