ItemHistory::getFields

public function getFields(): array
{
	if($this->fields === null)
	{
		$this->fields = [];
		if($this->item)
		{
			$this->fields = $this->item->getChangedUserFieldNames();
		}
		elseif($this->getId() > 0)
		{
			$list = ItemHistoryFieldTable::getList([
				'select' => [
					'FIELD_NAME',
				],
				'filter' => [
					'=ITEM_HISTORY_ID' => $this->getId(),
				],
			]);
			while($field = $list->fetch())
			{
				$this->fields[] = $field['FIELD_NAME'];
			}
		}
	}

	return $this->fields;
}