On this site https://www.aosabook.org/en/cmake.html section 5.3.3 mentions the possibility of developing plugins in C for CMake. Where can I find more information about this?
Plugins are loaded with
load_command(), but this command has been deprecated for many years due to the reasons outlined in the document you linked. It will be removed in the not-too-distant future. Writing new code that uses it will set you up for problems.