https://github.com/arduino/Arduino/wiki/Unofficial-list-of-3rd-party-boards-support-urls points to SparkFun: https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json whereas the readme in this github repository points to https://raw.githubusercontent.com/sparkfun/Arduino_Boards/main/IDE_Board_Manager/package_sparkfun_index.json One is master the other is main