There WAS supposedly a dead man's switch; numerous encrypted multi-gigabyte files have been shared over the years, and the theory was that their keys would be released if Assange or one of his associates didn't hit a button every X amount of time. But that doesn't seem to have materialized.

I don’t think any deadman’s switch has been confirmed by Wikileaks. They did however publish insurance files which are supposed to contain full unredacted archives of various leaks. One such insurance file was exposed when an either incompetent or malicious journalist at the guardian revealed the encryption key in a publication.

A deadman’s switch would be a specific mechanism to reveal such key. But the existence of it has to my knowledge never been confirmed by WL.