Problem/Motivation
Per #3498834: Dont use core's prepopulate function for core forms (Privacy) this violates GDPR, and it's also not secure on shared computers. In addition, modern web browsers have their own autofill functionality, there's no need for Drupal to re-implement it in JavaScript.
We should probably split several things that form.js does into their own libraries so that they can be attached (or not) separately.
Steps to reproduce
Proposed resolution
Because this is re-implementing a browser feature and it gracefully degrades if it doesn't work (because you need to have filled out the form before anyway), maybe we could remove it altogether in a minor release, rather than trying to figure out how to deprecate the feature?