almost 2 years ago - Shurenai - Direct link
Updating to A21 won't delete your world, But you won't be able to play the world on A21..

In order to continue your save you have a few options. One, you can just opt back into A20 when A21 is released. Alternatively, You can just copy the game folder currently from wherever you have it installed to another location, and launch the copy from the .exe in it's folder; That copy won't ever be updated by steam, and the copy managed by steam will be updated to A21 when it's time for that.