There is enough evidence for this atleast for schizophrenia.Multiple studies have suggested that schizophrenia will damage neurons on long run especially when untreated.
Diagnosis is still not straightforward and "objective" like other medical conditions but the day will come when we will be using some markers/imaging studies too to confirm the diagnosis.Often a psychotic episode might be diagnosed inappropriately as schizophrenia but when we evaluate the lonngitudinal history,talk to significant others and keep reviewing provisional diagnosis finally we come to a diagnostic clarity.
https://www.psychiatristahmedabad.com/2012/06/schizophrenia.html
