Maybe obsolete by "source" configuration

If you add a tag there that is already managed by another plugin you are shown an error. For example, if you add <h2> when the Heading button is in the toolbar you get this error.

The following tag(s) are already supported by enabled plugins and should not be added to the Source Editing "Manually editable HTML tags" field: Headings (<h2>).

This module is not obsolete as for the sites that I maintain we allow users to paste rendered html into CKEditor even though they don't have access to the 'Source' button. In that case we still want to restrict the html that they can use.