There seem to be lots of sleep overs happening for 9th birthdays for my oldest. She went to her first one recently. I know the parents well enough to know they'd look after her properly, and she was sleeping in a room with 4 other kids...I had zero concerns.
I'll only not allow sleep overs when I don't know or trust the family. My 7 year omd was invited to a sleep over last year...a single mum addicted to pain killers and she was getting a friend I didn't know at all to help out...and it was just my child invited. That didn't happen.
I will never ban sleep overs, but if I know the famiky.my kids can go. I loved sleepovers as a kid. We always had the best fun.