It's not an issue with the project itself, but it seems that in Arduino IDE 2.3.4 for macOS, the installation paths for hardware boards have changed. They are no longer located in the ~/Documents/Arduino/hardware folder. Instead, they now follow the Linux approach and should be placed in the ~/arduino-sketchbook/hardware folder.
This change impacts the instructions provided at https://github.com/rogerclarkmelbourne/Arduino_STM32/wiki/Installation. However, I’m hesitant to submit a PR to update the documentation because I’m not very experienced with this project and there’s a (big) chance I could be mistaken.