As Da Woods said, subinterpreters are already in the main release, so it’s as stable as the Stable ABI.
Free-threaded is an experimental build, because we don’t yet know exactly what shape things will have to be in order to work. Expect some breaking changes, or potentially an entirely new stable ABI (we’re discussing elsewhere whether to move to abi4 for all builds, to add a new parallel abi3t just for free-threaded, or just add a set of build-time options to choose which parts are stable). There’s a proposal that free-threaded is stable enough now that we can commit to its ABI, and so if that’s accepted then the new stable ABI can be determined, but until then we need to keep the flexibility to avoid designing in flaws and being committed to them for a long period of time.