I think Handbook 1: 13.6.23 "Special Situations" covers this.
Handbook 1: 13.6.12 covers adoptions, but in the wrong direction. But I think the message there is wait until it's final and you have as much information as you can get.
I see two possible outcomes if the adoption agency won't provide information about the new home, the child might be moved to "Address Unknown" with a notation of what's happened, or the record will be canceled - possibly sometime after they child turns 18 and hasn't been found.
If you can get information about the adoptive family, then the record can be moved into that ward. And I wouldn't rule out the possibility that the new parents will ask for the child's name to be removed from the records. Handbook 1: 6.14 outlines how that works.
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