LinkedIn just suggested to add to my network a GMail email address I created 8 years ago, and never used for any purpose (there are 3 mails on that account, all from Google). The only link between that account and my real account is an email I got from Google 8 years ago notifying me that a person invited by me created a GMail account.
I never gave a password to my GMail to LinkedIn. I suspect the way they got that contact is by LinkedIn Android app I briefly had installed on my phone two years ago, which somehow grabbed all contacts I had in GMail (the act I never consented to). Is LinkedIn mobile app known to do that?
The lesson: if you don't want LinkedIn to have all your contacts, the only way is to avoid LinkedIn completely.
In short, it just feeds an empty data set when the app tries to get your info.