Relax discouraged feature spec URL validation

[ddbeck]

In many PRs for discouraged features (example), we have add an old spec URL to the allowlist because browser-specs doesn't record it (for good and obvious reasons). This is less than optimal because the reason and URL are very unlikely to ever change (i.e., we'll never have cause to remove the URL from the allowlist).

We should consider relaxing the validation for discouraged features. Perhaps we can do something simple (such as periodically confirming that the URLs don't 404 or redirect) to confirm that the URLs are still, in some sense, "good" but not as rigorous as browser-spec's process.