On the face of it, this seems like a non-starter. If a particular immutable release represents a danger to the consumer (extreme example: the software contains a bug that could result in physical injury) one must have the ability to retract that release so that no further consumers of the software could be affected by it. It makes sense that a retraction of an immutable release should not be reversible in such a way that the release could be recreated with different contents. But retractions must be possible, for both ethical and legal reasons.
I would also argue that its not sufficient to simply apply a blanket "deny all" access control to dangerous releases (assuming such a mechanism exists), as this does not adequately convey the deprecating nature of the change (and as a result, could mistakenly be reversed in the future). Ideally the retraction itself would be immutable such that once retracted the release is inaccessible forever.
Now, it may be that all this is supported by the new feature; I haven't had the chance to test it yet. But nothing in the documentation makes this clear one way or another.