Raytraced Translucency just got a ninja update in the preview build of Unreal Engine 4.27, and Unreal Enging 5. Previously, in 4.26, raytraced translucency/glass didn't cast any shadows. At all. Now, it does!
I thought Raytraced Translucency was broken in the 4.27 preview update, but after talking to the devs at Epic Games, I found out that they revamped the way that Raytraced Glass Materials are configured. Now we actually control refraction with the refraction pin, as opposed to with the specular pin.
Thanks, Devs!