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?
The communists we should be mourning are the ones of the Tudeh party which were indeed executed by the Islamic government, although I'm not sure how much of a hand Raisi had in that. He is much more known for executions of MEK, which is not a communist group and is rather a US and Israeli-backed organization which has no real ideology other than opposing the IRI.