Look at almost any file in byterun, or any C file in asmrun. The convention is used for #defines, but since there are a TON of #defines, you end up having it all over the C codebase.
I'm not sure it's a bad convention -- it ends up being a lot clearer than ALL_CAPS. It's just different, and was jarring to me initially. There's also a mix of ALL_CAPS and Camel_snake being used for #defines, making it unclear which convention should be used.