Files that are "used in 0 places" (viewable at /admin/content/files) are not being deleted automatically by cron. They also show a status of permanent. How do I allow these files to be deleted by cron, or if not possible is there another way?
Prior to 8.4.x this was the default behaviour, but a number of issues with file reference counting meant it was possible for files to get deleted prematurely (and possibly also for stale files to linger, though that's not so important). In 8.4.x the default behaviour was changed: https://www.drupal.org/node/2891902.
For completion, to answer the question it's possible to change back to the old behaviour by setting
make_unused_managed_files_temporary to true in
file.settings. It's clearly not recommended (that's why core doesn't use it anymore) - see https://www.drupal.org/project/drupal/issues/2821423 to track progress on file reference tracking.