I do agree that in the past there will also have been children who missed out on being diagnosed with serious issues. In the past, it was probably easier for a child with anxiety/depression to fly under the radar, sometimes bunking off school, sometimes having a day off here and there- there was much less pressure on school attendance and children could perhaps deal with situations by avoiding them.
In the 90s/00s, there was more support in place for mentally ill teens. They could be given help (in some cases) before things got to crisis point. Now, CAMHS is massively underfunded and overstretched, and schools are also struggling to fund support staff etc.
There are also additional environmental factors- e.g. the number of children who live in rented accommodation on insecure tenancies. Regularly having to move throughout childhood can't be good for mental health. Children with parents who are stressed about money and maybe working multiple jobs or struggling with benefits changes- there are less people with "jobs for life" now. In some cases, I also think 50:50 split custody is not great for children- not having a secure home base is not great for children struggling with mental health issues.
I also agree that the new GCSEs don't help. They are very tough exams, and schools are judged on the results of every child- which means children constantly being pushed and judged against targets which can be artificially high. In the past, there were more vocational options for less academic children. We now have children who are expected to get a grade 3 at best being pushed through 8 academic subjects- this is not great for them at all, and it can make it hard for them to see a future where they might be successful. For some children with SEN, I also think that inclusion isn't always the best thing for their mental health.
In my day to day job as a teacher, I do see lots of children who are incredibly resilient despite really difficult stuff happening in their home lives.