After checking this website and other, I have found that these claims come from Radio Free Asia and others citing these so I am not sure. Haven't found good sources talking about this.
Source: https://moderntokyotimes.com/new-president-of-iran-linked-to-past-executions-of-communists-socialists-and-others/
The election result in Iran for a new president is a grim reminder that ultra-conservative Shia power concentration is in full swing. This concerns the new president being linked to the mass executions of 1988. These executions exceeded several thousands of people – some site tens of thousands – when the Iranian ultra-conservative clergy sought to crush all dissent. Hence, the electoral success of Ebrahim Raisi is bound to raise questions internationally.
Is this true or can I safely dismiss it?
I can find Persian sources for you, but it is true. He was known as the butcher of 88
Those sources will certainly help for this discussion. Thank you!