A UseBB 1 migration/conversion module for Drupal 8 has been released: UseBB2Drupal. This module performs migration of users, categories, forums, topics, posts and IP address bans without overwriting existing Drupal content. It also translates internal forum links to the new Drupal paths.
If you own a UseBB 1 forum and are willing to migrate to Drupal 8, please test the module thoroughly on a private or local environment and submit any issues to the issue tracker on Drupal.org. You can also clone the repo from GitHub and create pull requests.
Soon, this website will be replaced with a (probably read only) copy of the forum in Drupal 8. Existing UseBB 1 users are then highly recommended to migrate their forum to Drupal 8.