For PC players the single Player “Offline Mode” mod by hades is back in the steam workshop. I know it doesn’t satisfy vanilla purists or those questioning the incongruence between the playstation having a built-in offline mode and pc not, but it’s better than nothing.
Shiny! I didn’t unplug my internet connection, but I ran it with Steam in offline mode. Just a word of warning, due to database files that I don’t feel like messing with, apparently some users may have to create a new character. But still, it works.