It's a new keyword & bytecode feature, so there's no backwards compatible risks to it. Nobody's existing code would be impacted as nobody's existing code is using user-defined value types in the byte code.

It looks like there were some experiments and prototyping out of Valhalla from 2014-2018 (including a publicly available build: https://jaxenter.com/java-value-type-163446.html ), but there doesn't seem to be any updates since then