Unless the steam process shuts down unexpectedly, or you suddenly lose internet connection, or your computer reboots too quickly for Steam to keep up. Then Steam will decide it needs to ... validate something ... with the online servers before allowing offline mode. Far from idea.
That is up to the games - they can choose to hook into Steam's API for that DRM, or roll their own DRM (or both, thank you Ubisoft), or they can ship without any protection.
But for at least a few years, you've been able to launch steam without an internet connection.