Every week, Spotify updates your Discover Weekly & Release Radar playlists (respectively on mondays and fridays) – sadly it’s done by replacing your week-old songs with new ones.
Thankfully, you can use these two IFTTT applets to be 100% sure that you’ll never lose a song again.
PS. You can use these applets to archive other playlists (both dynamically updated by Spotify and Spotify’s users)