How to find unused or old twig template files?
It's difficult to find a deterministic method, so you probably need some kind of usage statistics.

A simple statistical method would be to clear the twig php cache:

\Drupal\Core\PhpStorage\PhpStorageFactory::get('twig')-&gt;deleteAll();


Then run the site for some time and check the twig php cache (/sites/default/files/php/twig) for twig files not having a compiled php template.