See though isn't that destruction of evidence? Which is pretty clearly illegal.
"Oh, you were looking for documents at my house? Sorry, I built my filing cabinet to shred and burn everything if I don't press a button every week" -- if you're arrested for (for example) financial fraud and you pull that kind of BS, you're in deep trouble. In fact, after you've started being investigated for something, any action to destroy potential evidence is super suspicious and may well be specifically illegal, even if (especially if) there's no way after the fact to prove whether it was incriminating or not. Just because you took that action far in the past when you set up the dead man's switch, doesn't excuse you from the consequences.
It's well established in corporate cases that people have an obligation to prevent automatic deletion of materials during an investigation. I'd be surprised if that didn't hold for private citizens too, why wouldn't it? If you would reasonably expect that the police have a lawful reason to search your documents, and you don't volunteer that those documents exist and are going to be deleted...I really don't see any ethical difference between that, and finding some way to delete them remotely yourself.
In a criminal case I don't see an ethical difference between "I did X and Y happened", and "I knew Y would happen if I didn't do X, and I didn't do it". Kind of like "I cut her brake line" vs "I knew her brake line was cut, and that she was going to drive the car, and I didn't inform her".
If there's an argument that a dead man's switch really is ethically different from active deletion...and I really do mean ethically different, not "more likely to get you off the hook for a crime"...I'd love to hear it.