Files that have no remaining usages are no longer deleted by default

I've been wondering why deleted images were accumulating on the file system, only to discover this is the intended behavior in Drupal 8 now.

It appears all images have STATUS Permanent, but those USED IN 0 places are not removed. Is the solution to modify file.settings.yml, setting make_unused_managed_files_temporary to true?

Also, moving forward, how is the novice Drupal user expected to remove image from their site. This needs some TLC, no?

if one decide to opt out from this new default behavior and set `make_unused_managed_files_temporary ` = TRUE. We are still left with old files that have 0 usage that are still market permanent and may not change their status to Temporary. Are they supposed to change their status? Is this something drupal would take care of? Based on my research I don't see anything that look for files that were marked permenant and set them to temporary based on the value of "make_unused_managed_files_temporary"
For now, do we have to do in a hook update once? or should this be a cron function?

Here's some PHP you can toss in an update hook in favor of the linked css:

/**
 * Move all managed unused files to temporary status.
 *
 * NOTE: Add your module name below, and change "N" to 8001 or above as needed.
 *
 * Implements hook_update_n().
 */
function YOUR_MODULE_NAME_update_N() {
  $query_force_temp_on_unused_managed_files = "
  UPDATE
    {file_managed} AS b1,
    (
      SELECT
        DISTINCT {file_managed}.fid AS file_managed_fid
      FROM
        {file_managed}
      LEFT JOIN
        {file_usage} file_usage_file_managed
        ON {file_managed}.fid = {file_usage_file_managed}.fid
      GROUP BY
        {file_managed}.fid
      HAVING
        (COUNT(file_usage_file_managed.count) = 0)
    ) AS b2
  SET
    b1.status = 0
  WHERE
    b1.fid = b2.file_managed_fid;";

  $database = \Drupal::database();
  $query = $database->query($query_force_temp_on_unused_managed_files);

  // Attempt to execute query, throw error if it failed.
  if ($query->execute()) {
    return t('Successfully flagged all file entities with no value in table file_usage for removal after 6 hours on cron run.');
  }
  else {
    throw new UpdateException('FUNCTION_NAME_HERE Query failed to execute.');
  }
}

Do the unused (removed/deleted) files ever get deleted? After setting make_unused_managed_files_temporary I see that files i have removed through the UI are now marked as status=0 in file_managed (even though Audit Files still states the status as permanent). Does this file ever get actually deleted; or is that up to me to write code to do? I would have expected file_cron to clean these up. Possibly only after temporary_maximum_age has expired? As minimum for this in the UI has a minimum of 6 hours (why?); i may have set it to 20 seconds by setting: $settings['temporary_maximum_age'] = 20; in my settings.php (even though there is no evidence this is overriden on the File Settings page (even with Override Warning module enabled).

I don't recall any of these issues in Drupal 7.

This is a problem for:
sites that want to maintain a media library

I don't quite get why this would be a problem.
A media is a separate entity, and a media referencing a file counts as a usage.

By default make_unused_managed_files_temporary in file.settings configuration is set to false

drush  cget file.settings
_core:
  default_config_hash: 0aMkoXYnax5_tHI9C9zHs-K48KJ6K75PHtD9x-0nbgM
...
make_unused_managed_files_temporary: false

As discussed in this thread, a file's status will remain "Permanent" even when its usage count is 0.

To change this and set the file's status to "Temporary" when its usage count is 0 and thus get deleted by file_cron (next time it runs, execute the following drush command

drush cset file.settings make_unused_managed_files_temporary true