Sprint website says 2019 VoLTE launch commercially.
https://www.sprint.com/en/support/solutions/services/faqs-about-calling-plus.html
Guess they started to work with easier devices that support VoLTE. Apple does not have a say since the IMS is controlled by the carrier. Sprint writes the configuration to allow it for iPhones and Sprint and Apple get it pushed out.
Me too, and by that I mean all iPhones from 6 and above.