UPDI
The programming and debug interface for newer AVR chips, squeezed onto a single pin.
Classic AVRs were programmed through ISP over four SPI lines plus reset, and debugged with debugWIRE or JTAG. Starting with the tinyAVR 0- and 1-series in 2016 and the AVR families that followed, Atmel — by then part of Microchip — replaced the lot with the Unified Program and Debug Interface: a half-duplex, UART-style signal on one pin.
Being a plain serial signal had a pleasant side effect. A USB-to-serial adapter and a resistor make a working programmer, and tools such as SerialUPDI and AVRDUDE support the setup. The trap is that the UPDI pin can be reconfigured as ordinary GPIO. Do that, and getting access back takes a 12-volt pulse — or a fresh chip.