D8 includes a feature to utilized the Advanced Content Filter (ACF) introduced in CKEditor 4.1. This would make it so that the configuration of the text format would affect the *input* of content into the editor, preventing any not-allowed tags from being allowed in the editor at all.

This has the advantage of automatically stripping naughty tags like FONT and attributes automatically added by browsers or MS Word. It can also provide functionality that substitutes for the normal paste-from-word cleanup.

Here's the D8 issue for reference: #1936392: Configure CKEditor's "Advanced Content Filter" (ACF) to match Drupal's text filters settings