They have been talking about missiles like this for decades now, but we don’t seem to be any closer to a deployable weapon than we were in the 90s.
I think in the past, the issue has been that its impossible to tell a nuclear ballistic missile from a conventional ballistic missile, so there is/was concern we could wind up starting a nuclear war when that wasnt the intent.
I think since that time, some arms treaties have expired or been let go, and also the threat environment is different. How the CONOP for such a system works without accidentally igniting a nuclear exchange with a competitor that may not even be the target of the conventional attack is something that would have to be hashed out.