This has only been fixed for new links. All existing links are still vulnerable.
From Google's Blog:
>"Today’s update to Drive takes extra precaution by ensuring that newly shared documents with hyperlinks to third-party HTTPS websites will not inadvertently relay the original document’s URL."
The bug has been patched so that any document with the "anyone with the link" permission will no longer leak its location in the referrer when someone clicks an HTTPS link in it. But it's possible that that happened in the past and the link was already leaked (and unfortunately, they can't exactly fix that). So if you have an old document, it's not vulnerable anymore, but it may at one time have been vulnerable so you might want to update it to have a new link (following the instructions in the Google blog post).
In essence, you needed to have a non-native document format uploaded to Drive without converting it (PDF is a good example); explicitly share this document with others using a particular setting ("anyone with the link"); and then preview it in the web UI and follow an outgoing HTTPS link (HTTP wouldn't be a problem).
It looks like Microsoft Onedrive did a similar thing too:
https://blog.onedrive.com/update-for-shared-links/ >"We chose not to disable all previously shared links, because the change only applies to a small fraction of shared files. If customers disable and then re-share a document, this will prevent further access to a document that might have been accessed."