Microsoft has been doing this for years.

They have an annual POLL online questionnaire. The answers are collated by 3rd party so by default you remain anonymous, unless you choose not to be. The aggregated results are granular down to a manager of 6 or more people. with plenty of space for you to write comments.

They have an annual performance review similar to those mentioned above. But at the same time, the employees fill out a manager feedback form that goes to HR & is aggregated to form part of their managers perf review.

Then 6 mths offset they have an annual "Career Planning" review. Very similar questions to your "Stay" Interview.

In most cases when the employee is given their final Performance evaluation & discussion both their Manager & their managers manager. is present.

This gives makes it harder for a poor manager to bury their people in order to hide their own incompetence. It also lets you see that the company takes your opinion & career very seriously.

While some are concerned that this feedback could be used against them. Most could see the high value in the process. In my mind it was hugely successful. Best on my experience & 100,000's of others over more than a decade, your idea of a "Keep" Interview has massive merit.