We've removed the Composer-managed vendor from the git repository.

There will not be any changes for people downloading Drupal 8 from drupal.org. The Drupal.org packager will add dependencies to zip and tar package.

If you're not using zip / tar files, e.g. when using a git clone, run composer install to get dependencies. See https://www.drupal.org/documentation/install/download#git for instructions.