In 20/20.5 I could set @Alpha to see a quick opacity preview in the viewport. It was nice for flipbooks, now in Houdini 21. It displays a value of 0 for @Alpha as something that looks like a facing ratio shader. Not sure what it's doing exactly.

Everything not red has an @Alpha of 0.0.

Attachments:
alpha_1.png (1.4 MB)
alpha_2.png (1.6 MB)
alpha_3.png (1.5 MB)

It does display correctly with no lights enabled:

Attachments:
alpha_4.png (1.5 MB)

ok, so enabling ambient occlusion makes it work correctly with other lighting modes:

Attachments:
alpha_5.png (1.5 MB)