While this looks good, I don't think it's feasible for a web crawler in most cases. Crawlers want to crawl a ton of URLs and it would have to make a request to your service for each and every URL.

What's the plan here? Check for a sitemap.xml (which generally only contains crawlable URLs anyway) or crawl the index and look for all links and send a request to your service for every URL before crawling it?

I personally think it would be better suited as a library where you can pass it a robots.txt and it'll let you know if you can crawl a URL based on that.