Current ThinkPads no longer have a physical serial port, but still have the logic integrated, all that is required is to attach the ThinkPad to a supported Dock or Port Replicator with a serial port.
ThinkPads produced from 1994 onwards have a NS16550A compatible uart.
What's the best thinkpad or best laptop with a serial port. Company I consult with has lots of RS-232 equipment and the prolific usb serial sometimes...