All the studies out there about the psychological impact of boarding on adults relate to people who went to boarding school 20+ years ago.
Boarding schools now are very different places. The majority of children weekly or flexi board and are at home most if not every weekend. Children have access to their phones and can phone home whenever they like once the school day is over. The teaching profession has become much more robust and rigorous about safeguarding, which has led to much safer and more supportive school environments for children. Boarding school boarding houses are set up very much like family homes, with a housemaster or mistress and matron available 24/7 and children having bedrooms that are cosy and filled with their own things.
As a teacher in an independent school, I can attest that the demands on children as they get older in terms of homework and extracurricular activities, coupled with often 1h + commutes to and from school, make them absolutely exhausted. They have very little 'quality' time at home in the evenings - most don't get home until 6 or 7pm if they've done an activity after school, then it's a quick dinner, a couple of hours of homework, then bed, before the whole routine begins again. Hardly time for lots of cosy chats and cuddles with mum and dad.
Most teenagers also love being with their friends far more than their family. Once they hit 13, those are their primary relationships. Getting to sleep over with your mates during the week is a lot of children's dream!
I will also say that teachers are in loco parentis during the school day. Speaking for myself and most of my colleagues - we do genuinely care deeply for our students, and we do get to know them very well. Of course we notice when they're not themselves and we do provide plenty of emotional support. It's also worth noting that often children tell their teachers things they would never tell their parents and we can help them negotiate tricky conversations with parents when their home lives are causing them distress.
Boarding school these days is often a very enjoyable experience for young people and many thrive in that environment. Any school can be an awful place for young people and cause them emotional distress, whether boarding or not. Insisting that boarding school is abusive based on stereotypes from the 1950s is really not helpful.