What happens if you loose Steam connectivity is this:
- If it happens while Steam is running it sows a "No Connection" text at the bottom of the Main Steam window.
At that point you can click "Steam" in the main menu and select "Go offline.."
You need to then confirm going offline and after that Steam will relaunch and go into the offline mode.
- If you lose connectivity without Steam running you're not out of luck.
You may be able to go into offline mode but it might depend on if steam was once started in offline mode
Steam crashing doing this is sad but not common. Also there's no pop up message anymore - that was ages ago. I have validated this again just now.
That said it's not such a big deal anymore. But it might depend on if you had steam switched to offline mode already or not, I'm not sure on this.
But it's totally possible to start steam in offline mode these days (gladly). This was different a few months ago.