Make one last release that is fully compatible with Drupal 8. This will allow anyone still stuck on Drupal 8 to have the latest code. After this, the development will cease on 8.x-1.x.
Change to semantic versioning and create a new 2.0.x branch and change the dependencies in the new branch so that 2.0.x-dev will not install on Drupal 8. Also, change the dependencies so that it will install on Drupal 10.
Make a 2.0.0 release and mark this new release as recommended.