Update: it is in the schema, but the Angular Language Service extension has not updated since the schema was fixed: github.com/angular/angu...
github.com
fix(language-service): add missing options to angularCompilerOptions … · angular/angular@7385890
…schema
The JSON schema contributed by the VS Code extension disallows unknown
`angularCompilerOptions` properties, so options missing from it are
flagged as "Property ... is not allowed"...