For me it is more like: "my manager sees that I'm looking for a job", and I really rather tell him that I will be leaving, as soon as I'm certain of a new job. It's none of his business before that point.
You better believe that if databrokers will buy information on whether you updated linked in that they'd also buy information on whether you gave the company a 1 star review.
Heck, you can even post your salary to glassdoor, so maybe your next employer would buy that information so they know the least they could offer you.
I remember reading a blog post by an employee that had gotten on the wrong side of google. When he came in their crosshairs, he said all his google machines forcibly updated themselves, and it became clear he was closely monitored.
I think the idea is that decent relationships have good boundaries, and proactively maintaining them is a worthwhile endeavor. This is especially important when there is a power relationship.
I've worked in management at companies with risk-based retention tools, and I've always seen them used as just that... retention tools. If anything, getting a high risk score as a high performer would usually be greatly in an employee's best interest, as it would be another justification to the higher-ups for a raise or better job assignment.
To be clear, I'm personally generally against these kind of panopticon data-slurp initiatives overall, I'm just surprised that the initial reaction is so strongly "my manager will use this to fire me" when I've only ever seen the opposite.