Allowing people to leave often has the opposite effect of what you would expect it to. Recently I was switching to a different Note app to keep all my notes in, and I realized that having the option to export all your data from an app actually makes me want to stay with it more. But feeling like I'm locked in or I can never leave a certain app. Makes me anxious and want to leave to another one for fear of not being able to leave later on.
The same is true of leadership, if you ask your people how long they want to stay with their ideal job is, etc. It'll actually make them want to stay more and not leave because they don't feel locked in.
I think trying to hold on to people, even if by policy, makes them feels claustrophobic.