It's in the Shepherd the Flock book , p. 112, par 4: "The announcement should be as follows: [Name of person] is no longer one of Jehovah's Witnesses." No scripture is cited. Of course it is for legal reasons, and of course they want to have it both ways.
This is one of the many cases of double speak that exists in the Watchtower. The announcement just says no longer a witness, but in the literature, especially the study literature designed for those who are already members, it will use "disfellowshipped" and "disassociated" all the time. They are programmed to know that the announcement equals one of those statuses. It also allows them to assert in court or in legal documents that they never announced that someone was "disfellowshipped" or even implied that they engaged in bad conduct, since you can't tell from the announcement if maybe the person just decided to pack up and leave. The juicy details will be saved for the congregation gossip.
Of course there is no scripture that merely says because someone decides to leave you have to shun them. This is no matter, however. They have been teaching that you have to shun DA's since the early 80's, even before the announcement change. So they didn't even have to try and back it up scripturally when they changed the announcement.