The game is not a "free-to-play" title, but there are legitimate ways to access it without a direct one-time purchase:

The game is included in the Xbox Game Pass library for both console and PC. If you have an active subscription, you can download and play it at no extra cost.

If you already own the original State of Decay 2 on Xbox or Windows 10, the Juggernaut Edition update was provided as a free download .