Drupal allows users to upload files to the site, which is usually done via file or image fields. Those allow users to configure upload location to be used for each individual field. Since this configuration variable was empty by default in the past Drupal often ended up uploading all files into one (root) directory, which slows down performance for sites with many uploaded files.
Drupal 8 now provides a default that provides a separate directory for each year/month of the date that the file is uploaded:
[date:custom:Y]-[date:custom:m]
By default, newly created file fields and the node article image field created by Standard profile use this default.
The default upload directory for user pictures was changed to:
pictures/[date:custom:Y]-[date:custom:m]
These are only changes to default configuration, so only affect new sites or new fields added to existing sites. Existing fields on existing sites aren't affected, and the ability to change the configuration value to something other than the default is unaffected.
