Issue
You've upgraded your Drupal 8 site to 8.8+ and now get the following warning in your site's Status Report.
Deprecated configuration
You are using deprecated configuration for the temporary files path. Your temporary directory configuration
matches the OS default and can be safely removed.
Cause
Drupal 8.8 changed some of the configuration handling for the temporary file path, moving storage of that path into a config system key. However, that key is not directly editable, and under certain odd circumstances can get populated with the default system temporary directory path.
Solution
Warning: the following is minimally tested, and while it seems to work, be cautious and consider testing on a copy of your site first. At a minimum, back up your database before trying this to be safe.
Putting the following into a module file temporarily and pulling up any page in the site should clear the value of the config key:
\Drupal::service('config.factory') -> getEditable('system.file') -> clear('path.temporary') -> save();
If the warning message then disappears from your Status Report, you can remove the above line from wherever you put it.