The 8.x-5.x branch is for Drupal 8.
The Webform 8.x-5.x module for Drupal 8 is a complete rewrite and was originally the YAML Form module.
A migration path from the YAML Form 8.x-1.x module to Webform 8.x-5.x will be provided.
A migration path from previous versions of the Webform module is still under development.
Learn more about the YAML Form module.