Nope, plenty of mental health issues in my high school class (late 90s).
One self-medicated with weed, alcohol and other drugs daily after his dad died suddenly.
One self-harmed and stopped eating because her parents were controlling and she was a closeted lesbian.
One self-harmed and was depressed because her father had untreated bipolar disorder, was threading the family but couldn't legally be sectioned for it, and her brother molested her.
One had the bad luck of both clinical depression and difficult relationships with his parents. He tried to take his own life three times before confessing to his mum what was happening and finally getting the help he needed. A friend of mine used to call him every day to check on him and make sure he was still alive.
The boy I dated in high school was misdiagnosed with a life-limiting heart condition and told he'd be dead before the age of 40. Also had difficult relationships with both divorced parents, and his stepparents, and his several half-siblings. He was obsessed with having control over a situation and over people he interacted with (including me). Anytime something challenged him emotionally, he had a "heart attack" that resolved itself without any hospital intervention. His symptoms were very real, but looking back, that was probably panic disorder.
I was undiagnosed ADHD and had both anxiety and horrible self-esteem, rooted in a lifetime of being told how difficult and dramatic I was. I had serious depression for the first time at uni.
TBH... I think at least three quarters of my high school class were either binge drinking, smoking weed every day, or both. The ~25% who weren't numbing with substances were anxiously overachieving at all the things. That is not indicative of a mentally content group of teenagers.
We had AIM and MSN Messenger and email chains back then. We had message boards on the internet. We had chat rooms. We had news websites. But that kind of interaction was limited to enormous computers in our living rooms or bedrooms, it was not carried around in our pockets every day. That is probably a key difference.
But I've also noticed that it's much safer these days for a teenager struggling with mental health to admit they are struggling. It's safer to seek out therapy and counselling. It's safer to ask the doctor for meds. And some of the media in their pocket has been excellent about educating them, and us, their parents.
My parents would have died of shame before they'd have let me go on antidepressants or see a psychiatrist. It would have reflected badly on them. People would have talked about how they'd failed me somehow. My friends and I seriously believed, growing up, that taking SSRIs would medicate our personalities away. So we ploughed on and found our own relief, for better and worse.