Why does wordpress add Theme not list a specific theme anymore?

Official repositories will omit themes and plugins that hadn’t been updated in two years from search and APIs. If you can find theme on repository site this is shown by following warning:

This theme hasn’t been updated in over 2 years. It may no longer be maintained or supported and may have compatibility issues when used with more recent versions of WordPress.

If theme is really gone and cannot be located on site then it was likely removed by author or repository team for some reason.