I use Steam on Ubuntu almost exclusively without internet. I've taken the following steps to enable offline mode:
Per this answer, created a file
~/.steam/steam.cfgwith the following contents:
BootStrapperInhibitAll=enable BootStrapperForceSelfUpdate=disable ForceOfflineMode=enable
Per this answer, modified the following lines in
~/.steam/steam/config/loginusers.vdfand made the file read-only:
"WantsOfflineMode" "1" "SkipOfflineModeWarning" "1"
While online, signed into Steam, selecting "Remember my password", then selected from the Steam menu "Go offline..." -> "Restart in offline mode".
Nevertheless after some time (2-3 weeks), when opening Steam I get the "Create new account / Login to an existing account" dialog. I have seen multiple claims that it should be possible to stay indefinitely in offline mode. How?