Playing on version 5.4.0 in a modpack and it seems framed double slab doesn't exist. Loaded an instance with just framed blocks and base forge and it still isn't there, so I'd assume it's just missing from the 1.18.2 version (5.4.0) .

Huh, it's a bit embarassing that I didn't think of that, thanks!

The double slab is not missing, it just doesn't have a dedicated block. Instead, it's built the same way vanilla double slabs are built by combining two single slabs. Same thing applies to the double panel.