Not available yet, but in the works. The link says that rates won't change, which should make Twigby pretty attractive for Verizon coverage.
This is good news: Twigby previously hadn't given any indication of how they would handle the T-Mobile/Sprint merger, so the wondering is over.
If you're using an older legacy-Sprint phone, you'll probably need a new one-- but you'll gain full Verizon service instead of limited talk/text roaming.
Is Twigby currently the only MVNO that still allows new activations on the Sprint network? I know that Tello allows existing active lines to continue but not new activations. RedPocket is unclear to me - I think they may allow for new activations but I think you have to provide your own SIM.
For most of us (budget conscious barrel-bailers), I think you're right. Prepaidcompare.net still lists some other MVNOs, but I haven't checked to see if they're still allowing new activations-- the price isn't attractive to me.