Mmm… I actually see it on my Windows install. See screen cap below.
Is it possible your screen scaling is such the “OFF” option is hidden?
FYI: the most current version of the game is 1.29.1. The current launcher version is 1.1.4. Refer to this thread to see current versions: What are the latest Zwift Game and Zwift Companion app version numbers?
Is yours up to date? 