Thank you. Yes, I was confused between the Jira Server vs Jira Cloud which is why I mentioned it!
I was in fact using PUT the whole time. Already confident that an ‘Unassigned’ assignee was possible.
I was using /rest/api/3/issue/{issue_id}/assignee and then using accountId as this documentation said name was deprecated (I was using the Cloud documentation considering my confusion as discussed already.)
It works changing it to /rest/api/2/issue/{issue_id}/assignee and sending in {“name”: null}
Appreciate the help!