When using Distant Horizons the normal intense fog/blindness that occurs when going through blocks in the phantom state doesn't appear. It's a bit glitchy, it shows the chunks near you and the chunks that are technically unloaded; basically allowing X-Ray

I made a datapack that 'fixes' the issue, by giving Phantoms blindness when phasing in to blocks.
It's not foolproof, since it only applies when you're more inside a block than the normal fog does, so if you really try you can still X-Ray, but it makes doing it unintentionally way harder.

I've been considering making being Phantomized always apply blindness, so the issue is avoided entirely (And free-ish invisibility being more dangerous), but eh.