UseBB 1 forums can be migrated to core Drupal 8 forums using the UseBB2Drupal module. This converts users, categories, forums, topics, posts and IP address bans to Drupal contents. Internal forum links are translated to the new Drupal paths. Migration can be performed either to a new website or to an existing Drupal 8 website.
Download the module from the Drupal.org project page after having set up a new or existing Drupal 8 website.
Alternatively, if you use Composer, execute:
composer require drupal/usebb2drupal
The README.txt file explains how to perform a migration and what contents is (and isn't) converted.
Bugs and support
You are welcome to submit patches on Drupal.org or create pull requests on the GitHub repo.