I @DannyS712
Do you know when this new magic word will be implemented?
On my side, I am ready to use it. All preparation task have been done.
Regards

@Niharika, @Framawiki,@Louperivois: Hi ! I’m back today with a new idea, probably easiest to implement.
I’m currently working on Special:UnusedCategories and there is a magic word doing the same kind of job I’m requesting:

It works, many thanks @Izno!

Hi! I was about making the same request for wp:fr because all TemplateStyles pages are now listed in Special:UncategorizedTemplates (almost empty on wp:fr).
Adding a documentation would be a good solution as we have a generic template for sub-templates pages self-generating a category.
If it is too complicated, adding a category in the TemplateStyles pages is also a possibility.
Many thanks.

@Niharika Hi ! First of all, sorry for answering so late but I’m really not familiar with Phabricator and this is the first time I'm posting a request.
Second, we already use this trick of using templates in a dummy page. The problem is that it cannot be used for most of the remaining templates I put in this category (sandbox, test cases, substituted templates, preload, KML code…). In addition, the advantage of using a category is that contrary to a dummy page or a whitelist it will be dynamic. No need of updating such a page.
Another trick I'm using each time it is possible is to use the template in its own documentation. Thousand of templates have been removed from this list this way.
As a workaround solution and if a category is not suitable, we could request a bot to periodically generate a whitelist from the category I created (and all sub-categories).
As a conclusion: Filtering options in all special lists would really improve all maintenance tasks.
If something is still not clear, do not hesitate to request me more information.
Regards