raspberry pi – UART Connector – Any open standard like STEMMA / quiic?

No, but feel free to use any pinout and connector you want by expanding on the Raspberry Pi standard.

The closest thing could be Digilent PMOD but it’s technically not a connector but just a pin header.

Basically take any USB-UART adapter with pinheader connector for VCC, GND, TXD and RXD as a reference. Some provide individually crimped connectors on each so they fit into any pinout.

Having a standard for this would require someone to come up with a connector, pinout, need to supply Vtarget for level shifting, and be intended for some purpose.

Most people just invent their own “standard” based on what they need or what their previous board had so they don’t have to invent a new one.