There may be a logical explanation, however currently I could not find documentation regarding this change.
In Drupal 7 you were able to set the public, private, and temporary files directory in the main settings.php, which made it very easy to alter the path of each directory per environment. However, in Drupal 8 this does not work/seem to be possible to change the temporary files directory path (so I wrote my first D8 module to support this)(and you can change the public and private path, but not tmp), but is there a good explanation of why this was left out or should you not make the tmp directory changeable in this way? Should the temporary files directory not be separated out by itself in D8 or put inside the private files directory (usually not a good idea)? Suggestions are welcome.