In my experience, the Steam offline mode only works if your computer is actually offline (without any network connection); if you're connected to the local network but the Internet connection from your router is down, it still tries to connect to the authentication servers while starting up.
Rockstar games require you to have an account at the launch iirc. And this us kind misleading, because while technically Steam has offline mode, but not necessarily the games you purchased on Steam. But having a unified UX is why I want to use the platform like steam to begin with.
It does offer an offline mode. It does NOT work with most games, because the publishers literally can't help themselves but add more layers of DRM on top of steam DRM and most of those these days require always online connections.