https://www.chromestatus.com/metrics/feature/popularity shows that the usage of -webkit-user-modify: read-write-plaintext-only is the highest among the three values of webkit-user-modify

If we deprecate -webkit-user-modify, we should provide alternatives to achieve the equivalent effect with contenteditable, which, unfortunately, don't exist. A workaround is to use <input> or <textarea>. However, the workaround can be quite convoluted, especially in the following use case:

Consider Chrome's DevTools, where we can inspect all elements of a document with the "Elements" panel. We can also double click at a tag name or attribute to edit it. The implementation is done by:
1. Each tag name and attribute is displayed with a <span> in the DevTools
2. When double clicking on such a <span>, -webkit-user-modify: read-write-plaintext-only is applied to it
3. When editing finishes, -webkit-user-modify is removed from the <span>

If we deprecate -webkit-user-modify, the transition between non-editable and plaintext-editable will be much more difficult and unnatural to implement.

Hence, we should standardize contenteditable=plaintext-only if we want to deprecate -webkit-user-modify