Ahrefs' Site Audit agrees that a page is “indexable” when it meets the following basic requirements:
It’s a HTML page
Search engines crawl and “see” your content in HTML format. To view how search engines see a certain website, you can enter a URL at: http://www.browseo.net/
It responds with a 200 HTTP status code
200 OK is the standard response for successful HTTP requests, indicating that the request was successfully received and processed by the client.
It doesn’t have a canonical tag to another URL
A canonical tag is a HTML element placed in a webpage, generally used to solve duplicate content issues. It implies that a webpage contains the “main” version of the content in question and points search engines to that particular page.
It doesn’t have any noindex meta tag
Including a noindex meta tag in a webpage’s HTML code tells Googlebot to entirely drop that page from its index, preventing it from appearing in search results.