Welcome, mike-o. Good to have you aBoard.

Having spent most of my working life in Child Protective
Services, I'll offer kind of an insider's view. Though not from the perspective of a resident, fortunately for me.

I believe the term orphan has slipped from common use because the term orphanage is not used any more, or only
rarely. And this, I think, is because the word orphanage
developed a well-deserved pejorative connotation. (This puts me in mind of the political correctness thread.) I have very little information on this, but I know that in the
fifties, there was something in the U.S. called the Orphan
Train, which was well-known for dispatching children to all sorts of places, and not all of them were carefully scrutinized to ensure that they really were orphans.
In the first half of this century, there was very little
oversight of who was sent to live in orphanages, or of the treatment they received once they got there. There was also very little checking as to the quality of homes the orphanage children were adopted out to.

Today, orphanages are "children's homes". A bereft child is not generally called an orphan, but is spoken of as being in the custody of a relative, or having been placed at a children's home.