And a manager that doesn't get that, and would retaliate/discriminate, is the kind of manager that probably has other, worse faults, and already engages in much worse anti-employee behavior.
Big-brain managers would use the information to ask the employee why they don't believe in the company anymore and use it as a feedback mechanism.
Galaxy-brain managers would take the signal from many different employees to find out if their assessment is correct or not, and figure out if it is time to cash out or to go long, just like every C-level executive does.
I would take the risk of manager discrimination (which I consider very low) in order to get better liquidity and avoid being illiquid for 8+ years or however long it takes a company to go public.
If employees sell most of their shares you gain zero insight from knowing their individual position vs looking at what the chart says about the entire stock unless, as rglullis said, you want feedback from those employees.
A manager that retaliates against employees basically ruins all three value propositions. Investors don't get their shares. Employees don't get their paycheck. You also don't get feedback. It's negative sum thinking.
A manager would only do it because they consider discrimination an end goal.