I haven’t seen a date for the general release. I have read on Steemitblog that they released it to Testnet, so I believe it’s in effect for Witnesses.

In order to avoid the RC issues and other problems, we experienced with HardFork 30, they are currently testing HF21 live within that contained ecosystem. Once the witnesses are satisfied that any unanticipated problems have been found and fixed they will release it to the rest of us. At least that’s my understanding of its current status. 👍