It's also possible that LI didn't share extra info at all with Scribd. The author takes a huge leap and assumes that all 500m LI emails have been transferred to Scribd because he got an email he doesn't recall ever signing up for.
* Never deleted an email from archive
* Don't recall ever registering on either ScribD or Slideshare
* Got the same email as the author, which is the first from either Slideshare or ScribD
* Could log in using my LinkedIn account and saw all my personal information from LinkedIn thereBy the way, my email is marked private on my linkedin account, but now it's on slideshare. So selling information you marked as private!
Doing so would in any case (I assume) violate the GDPR if LinkedIn doesn't doesn't have your informed consent for that. Perhaps their trying to weasel this under the "technically necessary" exception, but that strikes me as unlikely to hold water - at best they'd need to share some minimal token (e.g. if you sign in via google or some other auth provider, you get told what data google will share in advance, and it's possible to share fairly little).
The problem is that you might have "force opt. in" into a TOS agreement which states you allow the data transfer. So depending on the sign up with linkedln dialog it might have been legal...