I worry this ends up being a technical solution to what is ultimately a social problem. If the problem is that people feel threatened submitted feedback at their workplace, the issue is the structure of the workplace.
Yes, but it's possible for these structural problems to be invisible to the people who could change them, precisely because of the structure that's setup. There are definitely cases where the structure is there on purpose to create this sort of environment...and this won't do anything to fix those. But there are also cases where people are afraid to give honest feedback, but if they were able to do so in an anonymous way, management would either be pressured to, or would want to make a change.
That’s a fair point, but even if workplace norms are sane, the technical solution additionally protects against (say) a rogue IT admin gathering info in secret, or against future policy changes.