You can share or upload projects made with earlier versions of Scratch, and they will be visible and playable.

However, you can’t download projects made with or edited in later versions of Scratch and open them in earlier versions. For example, you can’t open a Scratch 3.0 project in the desktop version of Scratch 2.0, because Scratch 2.0 doesn’t know how to read the .sb3 project file format.