I don't see it either. Remember that the "new" Photoshop Lightroom CC is so new it is in version 1.
The replacement for the "old" Photoshop Lightroom CC is now called "Lightroom Classic CC" and it continues to have an option for a second monitor. Nothing was removed.
If you are accustomed to the tools, menthods and work flows of the "old" Lightroom CC we've had for years, stick with it and don't switch. Continue with Lightroom Classic CC.
I wish there was 2nd monitor... Adobe please fix.