Nick Sayer has created his own USB to serial converter:
There are countless devices to add a traditional DB9M serial port via USB. The general problem is that when you just buy a cheap one, you don’t have any idea what’s inside. With the reminder of the recent FTDI shenanigans, it’s become more important to use modem “class” chips that use generic OS drivers rather than proprietary driver devices. Cypress Semi’s CY7C65213 is one such device. Because it uses drivers supplied by the OS, there’s no opportunity for them to attempt to weaponize a proprietary driver.
Nick has also shared the board on OSH Park: