The language is a godforsaken monstrosity, but if you only have to write it once for some non-critical validation task it’s acceptable. For instance, I use JSON schema to (optionally) validate some generated JSON data which is fed into a web app as a JSON module. I also generated the TS spec for it from the JSON schema (although writing the TS spec by hand would have been 10–100x easier than writing the JSON schema). XML is obviously not suitable here — TypeScript doesn’t have XML modules.