Whenever I hold sneak and try and place a slab, it places it horizontal, not vertical.

This is happening on NeoForge 1.21.1

Vertical slabs are not currently supported in 1.21 versions. They are currently in active development.

Ah, got it, didn't know that. Because I read under features that vertical slabs were a thing. Nevermind then.

No worries, I'll make the description a bit more clearer but hopefully it won't be too long until I have finished adding the feature!

@CJMinecraft01 I noticed this has been marked as complete, does it apply to the latest versions?