Kind of? Let's say he has a detached header on another device and deleted the one on his phone. It's not at all destroyed now. Is what he did still illegal?

This is important because this is a feature of some crypto systems like LUKS. You can have your header on another physically septate device. If this is ruled on in an insufficiently nuanced way it could have very significant implications for carrying encrypted data without the key.

I think I misunderstood your original scenario. This one makes a lot more sense. Although I wouldn't want to be in the position of being the test case.

I dunno, now that you phrased it this way I feel like it does matter.

Image he took a blow-torch to a safe and the door welded shut. The contents are still there but the door is broken.

> Image he took a blow-torch to a safe and the door welded shut

This analogue isn't perfect, but it isn't relevant anyway: the laws in question criminalize acts based on their intent.

The blowtorch wasn't intended to destroy the contents of the safe (a la deletion of a key), but even still: if the intent was to impede or delay the investigation by welding the safe shut, it would be equally criminal.