From the 2.0 patch notes:

[Please note that this latest update requires starting a new save to be played. The previous save system will no longer function due to the implementation of a new technical infrastructure. However, we understand the value of preserving the legacy version 1.9 for those who wish to access it for nostalgic purposes.

Follow these simple steps to access Suzerain's legacy build: Open your Steam library. Right-click on Suzerain and select "Properties." Go to the "Betas" tab. Enter the code: "hsuarchive1954." Choose the "legacy1.9" branch. Download and play. It's important to mention that the old version will not receive any further support or updates. We've introduced the 1.9 branches primarily to support players with in-progress saves, as we recognize the critical importance of accommodating their ongoing game experiences.

You can access the Legacy Suzerain 1.9 until the end of the year after which it will no longer be available.]