Singleplayer menu says "Don't forget to back up this world before you load it in this snapshot"

I was going to play Minecraft 1.13 to see what was new and when I hovered my mouse over an icon that had versions lower than 1.13 it said "Dont forget to backup this world before you load it in this 'snapshot'" and I was playing on Release 1.13 not a pre release or snapshot.