> writing messages via in-game functions, like spelling words with dropped items or shooting walls I'm kinda ashamed I haven't thought of that.
And that just shows how little a mandatory backdoor policy would help: being able to read something doesn't mean you can understand it. It's always possible for someone to come up with a communication scheme that you didn't anticipate or can't interpret, and you can't legislate away that capability.
So a backdoor policy doesn't trade privacy for safety -- instead it trades privacy for a chance at safety, and pretty much just safety from careless, unprepared attackers.