We're using jsonnet for our systems and they have absolutely nothing to do with k8s. I'm not sure it's true to say it has ever gained much traction. It's just a niche case for complex configuration, and isn't the most publicised tool.
It does precisely what we need with zero fuss, cross platform and cross _language_ (we've embedded it in C++, .NET, and JVM executables).
We can use the resulting json config with a vast array of tools that simply don't exist for the alternatives such toml/yaml/hocon/ini whatever. In fact we tried to get HOCON working for non-JVM languages but there was always some edge case.
It does precisely what we need with zero fuss, cross platform and cross _language_ (we've embedded it in C++, .NET, and JVM executables).
We can use the resulting json config with a vast array of tools that simply don't exist for the alternatives such toml/yaml/hocon/ini whatever. In fact we tried to get HOCON working for non-JVM languages but there was always some edge case.