Antique is the original browser development version of Unturned created by Nelson Sexton in 2013. It is the first version of the Unturned, being much older than the Classic version. It was later replaced by the Classic version in early 2014, and it was brought back into use on April 15th, 2016. However, the developer stated that he will not update it ever again.
Servers are down, meaning that you can put any credentials in the login screen but you cannot play the game in multiplayer. You also cannot play the game without internet connection despite the lack of servers.
Its current version is v1.1. The only map is Skypoint.
You can play the Antique version by going to your game library, right-click Unturned and select Properties. After that, you jump into the Betas tab and in the dropdown menu, select '1.1 - Antique'. Once confirmed, an update awaits in the library where it will change the version into the Antique version.