You no longer have to settle for prepaid service if you're a T-Mobile customer hoping to use eSIM support. The carrier has expanded its eSIM support to allow use with postpaid (that is, regular) plans. You now don't have to be quite so picky when you're setting up a second line or freeing up your physical SIM slot for international cards.
The extended support is available now, although some existing caveats still apply. It only officially supports the iPhone XS and iPhone XR (and, we'd presume, future iPhones) right now. That's not surprising given the dearth of Android phones with eSIMs, but it's worth noting. You still have to go to a store to activate an eSIM. And like usual, you'll want to think carefully about which carrier gets the eSIM spot when only a handful of carriers have embraced the concept so far. It's an important step, though, and brings the US closer to a day when you might never need to open your phone just to start service.