Unreal Engine 4.26 Preview

Cannot use GPU Lightmass plugin with Preview 7. Crashes on launch. Is there no way to revert to preview 6? I really needed to experiment with this.
Also, the Github tree for UE4 seems… gone?
https://github.com/EpicGames/UnrealEngine/tree/4.26.0-preview-6

This link doesn’t work for me at all.