Hi, sorry for the late reply.
As you've probably already seen, in unstable mesg and write are already gone. wall still works, provided the caller is either root or wants to write to its own terminals. For bookworm, a stable update is in the works that undoes the mesg change. write(1) stays non-setgid, and sysadmins can make it setgid tty again, if they so desire. However, security bugs resulting from such a configuration will probably not see a lot of attention. Thanks for bringing this to my attention! Chris
