Really ? That's a bold claim. Can this string be hacked ? $6$WqQvgIwr$/BA/19FRJZf.z4buUwX5Sbls07ovv/lVZJ3V2En7VgTR4Skdpz98hFNuq3VX4nIaiSDKObO9jKbkPb5tdt7zY1
Eventually :)
I guess the point is that you cannot protect anything from the future if you don't just destroy it completely.
On that note, the matrix.org server currently doesn't delete messages from rooms that have become empty. It probably would be a good idea to do so.
Does synapse allow that for self hosted?
Yes, IIRC it's a script you have to run periodically.
"Everything" clearly means "Every x for which 'x can get hacked'" is not a type error.