Yahoo! Search cannot guarantee inclusion in the search index
This article explains that Yahoo! Search cannot guarantee that all websites will be crawled and indexed for inclusion in the search index.
The goal of Yahoo! Search is to discover and index all of the content available on the Web. We continually strive to provide the best possible search experience. Our database is updated daily to capture newly created and changing pages. And even though we crawl billions of pages, we cannot guarantee that all pages from any given site will be included in the search database.
The organic search results on Yahoo! Search are now powered by Bing. If your site does not appear in the Yahoo! Search and Bing results, you should first submit it for inclusion.
Once submitted, please allow several weeks for your content to appear in the Yahoo! Search and Bing results.
If you have submitted your site in the past to Yahoo! you may still have to submit it to Bing. Please consult this Bing help page for further assistance.
You may also want to sign in to the Bing Webmaster Center tool. You can use this tool to improve your site's SEO, submit your sites and XML-based Sitemaps to Bing, get data on which pages of your site have been indexed, backlinks, inbound links and keyword performance.
NOTE: Yahoo! is unable to provide any status updates or assistance regarding submissions to Bing.