Some wifi controllers can also boot like that. In particular ESP8089 chip that shipped with some android tablets circa 2012-2014.
Later, Espressif took that chip, modified bootrom to be able to boot from an SPI flash as well, and marketed that variant as "ESP8266". Serial bootloader was kept as a debug/programming interface, and that was inherited to ESP32 and later chips. All of which can boot directly from serial.
Later, Espressif took that chip, modified bootrom to be able to boot from an SPI flash as well, and marketed that variant as "ESP8266". Serial bootloader was kept as a debug/programming interface, and that was inherited to ESP32 and later chips. All of which can boot directly from serial.