Right-click the game in your library > Properties > Betas > Select "temporary_1_31" or "temporary_1_35".

By the time Version 1.35 arrived in 2019, the engine was undergoing significant changes. This update was famous for:

You do not need a new key to play 1.31 or 1.35. If you own the game on Steam, you can "time travel" to these versions for free using the Beta Tab in the game properties.