There are obviously very few relinquished babies now, thankfully the stigma of being an unmarried mum has for the most part gone. Most adoptions are of children removed from their families by Social Services. That is the case with my DDs. We have a life story book for each of them provided by their social worker, which we share with them. There are also 2 later life letters, one for the child to read when at primary school age, and the other one for when a teenager, with far more information.
It's a tough thing for adoptive parents to do, to bring it up, I suspect most adoptive parents in the past just kept putting it off, not because they were selfish (although some were, no doubt). It's easier for us as our 2 DDs are full birth sisters.
My DNephew OTOH is a relinquished baby, probably for the simple reason that his birth mum didn't realise that she was pregnant before she was past the legal limit for her to be able to terminate the pregnancy. He was her fourth child and one of her children has SN. So she had the baby secretly and relinquished him for adoption and never told anyone in her family about it. So her DC don't even know that they have a younger sibling. She wouldn't even say who the father is.
This will be a heartbreaking thing for my DSis and DBIL to explain to my DN. They have a life story book for him, but obviously it doesn't include those distressing details.