Delete "permanent" file when it has 0 uses 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 behavior, 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 behavior was changed. See Files that have no remaining usages are no longer deleted by default.
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 Dealing with unexpected file deletion due to incorrect file usage to track progress on file reference tracking.