It's up to the server to use proper headers. Why say a file is ok to cache for years if it actually isnt? If the same url will change content then use shorter cache times or require active revalidation and/or etag checks - or just use the typical cache busting querystring parameters.
This "immutable" flag is unnecessary.
From the cache control rfc:
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
Clients SHOULD NOT issue a conditional request during the response's
freshness lifetime (e.g., upon a reload) unless explicitly overridden
by the user (e.g., a force reload).If the implementation is faulty, what is another spec going to solve? Again, there is no need for an "immutable" flag because existing cache headers already express everything that's necessary.