Quoted from 7dtd forums:

Hello Survivors!

With the Experimental phase of A21 coming soon, we want to patch an issue we found during testing.
This patch only addresses a benign save game compatibility error users would encounter when returning to A20 after a session in A21 Exp.

This patch does not contain any content you may have seen in an A20.7 Twitch build, those features will roll out with A21.

Please remember A20 savegames will not work with the A21 version, you will have to start anew.

Stay alert for developer stream announcements to catch the latest news about A21!
You can find out more about streams in our forum.