From Houdini 18.5
Will the free Houdini Engine Plugin for Unity/Unreal license work with any version of Unreal or Unity?

This license will work with Unity 2018 LTS, Unity 2019 LTS, and Unity 2020 and Unreal 4.25 and 4.26. It is possible that earlier versions will work, but we only offer support for the current version + two previous LTS versions of Unity.