This is for the endpoint /rest/api/3/issue/{issueIdOrKey}

e.g. Suppose I update an issue with JSON like this:

{
  "fields": {
    "assignee": {
      "name": null
    }
  }
}

or

{
  "fields": {
    "assignee": {
      "id": null
    }
  }
}

Both of these should cause the assignee to be unset. Currently if you use the "id" form you get an error message about the name field being invalid.