As others have noted, the sentence is potentially ambiguous.
"The latest issue of the magazine is out" means that it has been published.
"Bob has been caught stealing from the company. He's out" means that he has been fired.
That is, "X is out" could mean "X is released to the outside world", or it could mean "X has been thrown out, i.e. rejected, discarded, or expelled".
In context, I think the boss probably means that the rule has been rejected. While we often say that something that one might distribute to the outside world, like a book or magazine, "is out" to mean it has been published, this meaning is rarely used in other context. If the boss has said, "The new rulebook is out", I'd take that to mean "published". But "The new rule is out" most likely means "rejected".
But people can be sloppy in their speech and writing, especially in emails, so on something like that it doesn't hurt to ask for clarification.