Override twig template file from classy theme
As discovered through the comments discussion the answer in this case is to place the template into the admin subtheme (not the default theme). 

When adding new templates to your subtheme it's important to be aware of the fact that there are usually two active themes in any Drupal site: the default one for "front-end" and an admin one for the "back-end" side. 

There is also a part of the site that is ambiguous about whether it falls under "front-" or "back-end", and that is Editing content pages. 

There is a setting under Appearance that controls which theme should be used for editing content. When adding a template for content-editing pages it's important to double-check which theme is chosen there to know which theme to add the template to. 

As always, don't modify the contributed themes, instead add a subtheme that extends them and add the template there. Then adjust the Appearance settings to use the subtheme.