The Soviet system used psychiatry as a weapon by diagnosing political opponents as mentally ill in order to confine them as patients instead of trying them in court. Anyone who challenged the state such as dissidents, writers, would-be emigrants, religious believers, or human rights activists could be branded with fabricated disorders like sluggish schizophrenia. This turned normal political disagreement into supposed medical pathology and allowed the state to present dissent as insanity.
Once labeled in this way, people were placed in psychiatric hospitals where they could be held for long periods without legal protections. Harsh treatments were often used to break their resolve. The collaboration between state security organs and compliant psychiatrists created a system where political imprisonment was disguised as medical care, letting the Soviet regime suppress opposition while pretending it was addressing illness rather than silencing critics.
I went through the article, and the highest claims are of 15k people over the 70 year history of the Soviet Union. For reference, there are over 700k homeless people in the USA by the lowest estimates.