Several years ago I heard the company I worked for say they had a way to get notified if it seemed like an employee might be thinking of leaving, so they could take some kind of action. I now wonder if LinkedIn, or various job sites, were selling them data.

LinkedIn might not need to sell the data. You can set your profile to “open for work” privately, and only recruiters can see it. So if your company has people with LinkedIn recruiter accounts, they could see your profile set to looking for work

PS: I guess given that recruiter accounts are paid, LinkedIn is technically selling access to the data in a way