Clear all messages in a private DM

There have been posts about this previously but I'm sending in a new one to show more support. This has been suggested a lot. It would be nice to have the ability to delete all messages in a private DM. Maybe both users would have to 'agree', and the DM's are cleared. Why doesn't this exist yet anyway, considering it can be done with servers? I wonder how hard it would be to implement and if there's a reasom it hasn't been yet.