>I strongly suspect that it’s not new in the US either, or at least was only unusual compared to a status quo that existed only for a blip of time during the 20th century.
You're entirely correct. I'll go a step further and say it never even went away--at least in my experience as a child of lower middle class White baby boomer parents in the South.
I have a great uncle on my mom's side who stayed at home while he finished college and then him and his wife lived with his parents for a while after he graduated.
My dad's oldest brother and his wife lived with their parents for a while.
My dad's youngest brother lived with his parents for years.
My grandparents eventually moved in with my dad and his wife--partially because they were getting too old to maintain a house, but also to help him afford a new house after he'd gone through a bankruptcy.
Both sets of grandparents also provided my parents with free childcare, and my parents and aunts and uncles all borrowed plenty of money from my grandparents over the years.
I have many friends who've had similar experiences.