This past Sunday I did a livestream where I tried out the new issue forks functionality on Drupal.org. The feature is currently beta and project maintainers need to opt-in for the functionality. At the moment you can create a repository fork for an issue, create a work branch, and commit to that branch. You will need to manually create a patch to be uploaded, as Merge Requests with Drupal.org issues are still being worked on (the Drupal Association Infrastructure team is hoping to have this available for DrupalCon Global!)
When you view an issue that supports issue forks, there will be a new block in the sidebar. This allows you to create an issue fork or view information about the existing fork and its branches.