Reports on reddit show that the latest update to Grand Theft Auto V on Steam has disabled the ability for players to play offline, even in singleplayer mode.
According to Rockstar Games support page, this is an unintentional bug:
This issue has been interpreted as a way of forcing an always-on DRM onto the game. We look forward to seeing a timely update which fixes this issue.
The changes coincide with the launch of Rockstar Games Launcher, now a required part of installing Grand Theft Auto V on Steam. The new launcher includes a number of dependencies that need to also be installed: