Should this file also have information about obsoleteIn version in addition to deprecatedIn version?

The code generators support an obsolete property under dart-sass (see the example in the comment at the top), but there aren't actually any obsolete deprecations yet.

It might be better to define the key with an empty value in yaml. This way we don't have conditional key in schema, and will making paring logic easier and more consistent in other programming language. In some programming language, the yaml object mapper may require the key to exist.

E.g.

call-string:
  description: Passing a string directly to meta.call().
  dart-sass:
    status: active
    deprecated: 0.0.0
    obsolete:

Another question I have is that how do we make sure the host would use the same version of deprecations.yaml if the host is not build at the same time as dart-sass? Should this be versioned / tagged like the embedded protocol?

We only have a single primary implementation of sass. Other third party implementations may not necessarily implements the same language features, and thus may not use same list of deprecations. It is a bit awkward if third party implementation has to update sass/sass repo for their own deprecations that may have nothing to do with dart-sass.

It's probably better to removing the "dart-sass" nesting in the yaml, and put this file directly in the dart-sass repo so that the file is tagged together with dart-sass version? For third party implementations, they can provide their own deprecations.yaml in their own repo if they wish.

I don't think versioning the YAML file is necessary, since an embedded host could always just ignore any Dart Sass versions in deprecated or obsolete older than the compiler version they're using.

The deprecations list is also part of the JS API, so it makes sense for it to live here, rather than in sass/dart-sass; we still want a dart-sass section because those versions aren't specified in the JS API, as some other non-Dart Sass implementation could support the JS API in the future, and it could use the same language-level deprecations but with different versions than Dart Sass.

I agree with @jathak. Deprecations generally represent behavioral differences between versions of the specification, and so can be expected to be fairly consistent across implementations other than version numbers. I think having some edge cases where we may need to add implementation-specific deprecations that aren't necessary in other implementations is a fine price to pay for having deprecations in general be a well-defined part of the JS API.