https://doc.wikimedia.org/cover-extensions/ provides coverage information about various repos. For every repo the coverage is available in machine-readable Clover format (although not linked from the web interface so one has to guess that), but the only way to get the list of repos seems to be scraping the HTML. It's easy (the coverage tool does it), but it would still be nicer if there was a machine-readable list which does not break whenever the HTML template changes.
I am not sure what you are looking for. Do you need a machine friendly list of (extension, coverage percent) ?
The code is in integration/docroot.git , the root page is org/wikimedia/doc/cover-extensions/index.php which relies on shared/CoveragePage.php
Maybe it be can be enhanced to support outputing a json/xml.