Is there any API for changing player nickname on TAB menu and over player’s head?
Also, why Cuberite sources on GitHub have branch 1.14?
1.13 and higher changed the way blocks are stored which is a huge task. That's why the branch hasn't been merged yet. I believe that branch is actually outdated, the one to look out for is the Block State pull request:
https://github.com/cuberite/cuberite/pull/5151