ISIS did this:
Meet on private online server for 1on1 pvp game.
Shoot letters in walls for messaging.
Exit server
Discussion
wooden signs in minecraft would
work.
You can set a server-side max value for things like bullet-holes, so they just dissapaer automatically when more impacts are happening,
They also used the email-draft-scheme, but that was insecure because of nsa-compromised email- servers/hosters.
Impacts are just coordinates in space, no real letter will be saved on any device ever.