Various (theming) development-related overrides used to live in settings.php, even though they actually change parameters for some services. Symfony's container supports defining parameters and therefore it's silly to keep "settings" that are transformed into "container parameters" inside settings.php if we can move them to a site-specific services.yml file.
Site-specific services.yml files (so sites/example.com/services.yml) are already automatically picked up (so nothing new there), all we did was make settings.php simpler and smaller by moving them into this services.yml file, which is a more logical/native place for them: no transformations are necessary anymore.
So e.g. the twig_debug setting no longer lives in sites/example.com/settings.php, but in sites/example.com/services.yml — the installer automatically creates that file (which sets the container parameters to their defaults just like before).
Gradually, we can hopefully make settings.php simpler and simpler, until the bare minimum is left.
Instead of one file being written to sites/example.com (settings.php) there now are two. Scripts — for example for reinstalling Drupal for development — will need to be updated accordingly!
