STM8EF-MODBUS deserved a GitHub project, of course with Travis-CI build automation, and docs.
Some of the properties are rather cool, e.g. the modular architecture and the clear separation of concerns between MODBUS protocol and other application logic :-)
Right now, on Aliexpress, the "MODBUS-RTU 4 way relay module STM8S103 MCU" is available at a good price: $6.65 incl. shipping.