With Minecraft RTX coming, I believe that the "mechanics" for light SHOULD have parity as much as possible for both RTX and NON-RTX.

Therefore:

Slabs and Stairs SHOULD block light on vanilla.

I never understood why a roof made out of stairs or slabs lets through all light. As long as there are no holes, light shouldn't pass through, no matter RTX or NON-RTX. : )