Leave no phone behind... that's certainly not any carrier's motto. T-Mobile at least does a better job than some others when it comes to long-term support. The LG G2 is getting a 617MB update to Lollipop today, just a week after the G3.
The OTA will start showing up on phones over the next few weeks until it's on all devices by May 22nd. You won't be able to force the update—it'll happen when it happens. Although, I bet LG's mobile support tool will be able to sideload it.
T-Mobile doesn't list a changelog for the Lollipop update (build D80130b), but you should have a good idea what it includes now: new lock screen, UI tweaks, smart lock, and so on.