The "get link" option works the same way as unlisted YouTube videos—it's not really private but, theoretically, not quite public, either, since the link needs to be publicized somewhere.
The secret sharing links are really just security through obscurity, and it turns out the links are actually guessable.
Along with Drive, Google is also changing the way unlisted YouTube links work, and the YouTube support page actually describes this change better than Drive does:.In 2017, we rolled out an update to the system that generates new YouTube Unlisted links, which included security enhancements that make the links for your Unlisted videos even harder for someone to discover if you haven't shared the link with them.Thanks to account-based sharing, anyone who accessed your unlisted Drive links in the past will still be granted access to them, even if you upgrade the security.
Not letting third parties create a list of all your unlisted files is a good thing, but don't confuse this link change with any actual security.You should never share anything over the "unlisted" or "get link" features on YouTube, Drive, or Google Photos if you actually want it to be private.