STM8 eForth supports a range of boards. and provides a method for building new ones. The problem is that the new board needs to be added to the STM8 eForth repository or a fork of it.
This has changed with the pre-release STM8 eForth 2.2.24.pre1: the source code is part of the release archive, and maintaining a board configuration in a "downstream" project is possible:
The STM8 eForth MODBUS repository now has a first "board folder", and building a Forth binary for the STM8S001J3RS485 board is as simple as cloning the repository and running the following:
make -f forth.mk BOARD=STM8S001J3RS485 flash
Of course, you also need to install STM8FLASH and the SDCC tool chain (or use a docker-sdcc).
Work on variant build in downstream projects, and auto-build of STM8 eForth isn't complete (providing binaries with MODBUS code needs some chainges), but the main building blocks are in place.