[2021] Start new game before Alpha 20?

And nothing is preventing you from staying on the current version/save after a20 drops. The developer even provides the ability for you to do that. When a20 goes experimental, the current stable will appear in the Betas tab for you to choose.

Don't stop your fun because of what might happen in the future. That's just stupid.

I can guarantee than an a19 save will not function with a20.