I just created a new branch in the User Guide git repository: 8.x-6.x, which corresponds to Drupal version 8.6.x.
Please make all future commits and issues in this new branch. Thanks!
Also note that just before I made that 8.x-6.x branch, I rebuilt e-books, committed them to the 8.x-5.x branch, and made a new 8.x-5.1 release. So, if you anyone downloads e-books, they will be reasonable current. Sometime soon I will make an 8.x-6.0 release as well.
--Jennifer