It isn't something limited to supporting adopted children either. I recall a few years ago a clinic that treated anorexia nervosa through essentially taking people back to bottle feeding and then weaning them. Including spoon feeding, heaps of praise, rocking them etc.
I can’t imagine that would in any way be a clinically appropriate way of treating anorexia. I’d be interested in any research supporting that view.
In terms of children, I think this is one of those things that’s git a touch of the Chinese whispers about it - eg the idea is based on sound theory about raising the child you have based on their developmental age, coupled with the knowledge that infants tend to regress following significant change ie in terms of feeding, toileting etc which turns into “treat them like they’re younger” as a short hand.
What I mean is, like so much in adoption, there’s been a conflagration of a few different concepts we know through research which is distilled into very simplistic advice where 2+2 =4, or not.
I think it’s good sense to parent the child you have, who may present older in some areas and younger in others, and to offer them nurture in a way that feels accessible to them. I won’t however be taking my 9 year old back to bottle feeding.