After a good gap, it's that time again. The demo and full game for Rock Band Blitz will be coming down on or before August 28th. Released in 2012 for the Xbox 360 and PlayStation®3, it's not backwards compatible on either Xbox One or PlayStation®4 (nor will it be in the future), but all 25 songs on the soundtrack can be brought over to Rock Band 4 at no additional charge. The steps for that process can be found here.
Speaking of the songs, all 25 songs featured on Rock Band Blitz were released as standard DLC after the game came out. Those versions of the songs will remain up until further notice.
tl,dr - Songs are safe, game and demo are going.
25 songs for $15 is a pretty good deal if you haven't picked this up yet.