Would love to see examples for Rust, Go, NodeJS, C# and Java so that I can add them to my GitHub stars. I could find a few results for XDG support in Go, Rust, and NodeJS, but none of the aforementioned languages supported standards across all of macOS/Windows/Linux. Hopefully it's just my Google-fu failing me and people can correct me in the comments?
C and C++ still have strong roll-your-own culture, with the exception that C++ now has a decent filesystem module in the standard library but still no standard config file parser AFAIK, everyone usually imports nlohmann's JSON library.
Many of the programs complained about are written in C. I'm sure that there are libraries for c that abstract this, but there isn't one that is common enough that most c programs can depend on it.
E.G: for python, the current go-to pypi module for this is platformdirs (https://pypi.org/project/platformdirs)
It's probably easier to promote the use of the module than explain the standard.