system/Database/Forge.php
public function dropForeignKey(string $table, string $foreign_name)
	{
		$sql = sprintf($this->dropConstraintStr, $this->db->escapeIdentifiers($this->db->DBPrefix . $table),
			$this->db->escapeIdentifiers($this->db->DBPrefix . $foreign_name));

		if ($sql === false)
		{
			if ($this->db->DBDebug)
			{
				throw new DatabaseException('This feature is not available for the database you are using.');
			}

			return false;
		}

		return $this->db->query($sql);
	}
public function dropForeignKey(string $table, string $foreign_name)
	{
		$sql = sprintf($this->dropConstraintStr, $this->db->escapeIdentifiers($this->db->DBPrefix . $table),
			$this->db->escapeIdentifiers($this->db->DBPrefix . $foreign_name));

		if ($sql === false)
		{
			if ($this->db->DBDebug)
			{
				throw new DatabaseException('This feature is not available for the database you are using.');
			}

			return false;
		}

		return $this->db->query($sql);
	}
Foreign Key Drop

@param string $table        Table name
@param string $foreign_name Foreign name

@return boolean|\CodeIgniter\Database\BaseResult|\CodeIgniter\Database\Query|false|mixed
@throws \CodeIgniter\Database\Exceptions\DatabaseException