Update April 10, 2017: Well that was quick. It looks like Jay Z's catalog has returned to Apple Music, though it still seems to be absent from Spotify.
It seems that rapper Jay Z, also known as Shawn Carter, has removed most of his work from Apple Music and Spotify. All of the artist's solo work seems to have disappeared from the services, with only collaborations with other artists still available. It should be noted that Jay Z is a co-owner of Tidal, a competitor to both Apple Music and Spotify.
As noted by MacRumors, there has been no definitive explanation given for the removal at this time:
Jay Z co-owns Tidal, a rival streaming music service, so his decision to remove his discography from Apple Music could be for competitive reasons. Kanye West, another stakeholder in Tidal, also implied that there might be some animosity between Apple and Jay Z in a series of tweets last year.
Jay Z's albums appear to have been removed from Spotify recently as well, however, so his decision might be unrelated to any potential feud with Apple. Spotify on Twitter today said it can "confirm that some of Jay Z's catalogue has been removed at the request of the artist," but it did not provide a specific reason.
At this time, Jay Z's catalog remains available for purchase on the iTunes Store. It seems likely that in the current streaming music environment, services look for any competitive advantage that they can find, and Jay Z, a popular artist with a large following, may sway some customers Tidal's way.