I think it may be difficult to get an exhaustive list of all possible files which need to live at the root. For one thing, different content management systems may place various files at the root, while others may place those elsewhere, so it depends on what platform you’re using.
index.html file will be found at the root, but keep in mind that your root is still a folder, which may have different names, depending on your web host or your CMS.
Then, you have changing standards. The
sitemap.xml file used to be commonly be placed at the root, but these days many CMS’s like WordPress (via plugins like Yoast) allow for creation of a
sitemap_index.xml file, which then leads to a list of sitemaps broken down by content type. Sometimes, they all live at the root; other times, they’re in a directory. Having them in a directory is okay, as long as the sitemap index file is at the root and the search bots can easily find and crawl that directory. Thus, the
sitemap.xml file may not exist on a website at all anymore, replaced by a (slightly) more complex sitemap information architecture. More on WordPress XML sitemaps here.
And then you have specific use cases. If your website is a publisher and actively sells inventory for ads to display on, you need an
ads.txt file. This file should be at the root. If you’re an ad exchange or an SSP (sell side platform), you need a
sellers.json file, which should also live at the root. Read more about ads.txt and sellers.json.
Perhaps the best way to go about it is, learn more about your CMS, figure out the functionality you’re looking for, and follow the standard, where the documentation will tell you where the crucial files should live.