Update: Well, it seems as though someone pushed a button they shouldn't have, and the page has been updated to remove all mentions of a KitKat update. We'll be on the lookout for the actual update coming soon.
The LG G Flex on T-Mobile receiving an update today to Android 4.4 KitKat. The update brings the G Flex up to the most recent version of Android (that'd be 4.4.2, not the 4.4.3 erroneously listed on T-Mobile's update page), as well as improving performance and battery life and bringing LG Knock Code to the G Flex.
The update is pushing out over-the-air for G Flex users and weighs in at around 150MB (so we suggest you download it over Wi-Fi).
Go check your T-Mobile LG G Flex for available updates, tap the menu key on the home screen, tap System Settings, select the General tab, tap on About Phone > System Updates > Check Now.