Today I learned about "foldering"[1], which uses the drafts folder of email systems as a "dead drop" between multiple participants. It goes back to at least 2005.
Back when I was in IT, circa 2010, I learned that several of the users of our systems routinely relied on the "deleted items" folder of Microsoft Outlook as a filing system, with multiple gigabytes important files stored there. 8(
Our customs are routinely ignored by everyone else.
The Spy's Son: The True Story of the Highest-Ranking CIA Officer Ever Convicted of Espionage and the Son He Trained to Spy for Russia
Then these people didn't change their ways when they were given adequate space.
Former head of the CIA, David Petreaus, was using the drafts folder of a shared Gmail account to communicate with his mistress.