“Dissapearing message” is a group/chat setting! Everybody can change again.
Thanks guys! Sounds like that timer setting could result in a bit of a ping-pong game then, but at least nobody can mess with your inbox silently.
< disappearing message turned off >
Actually such is fine, using (thought of it) before sending messages, as there could be “non offtopic” or “usefull” ones one would like that all keep.