By 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...