WordPress 4.7 is now in development. In this version the team decided to rearrange some of the toolbar buttons in the TinyMCE editor. The headers drop-down menu is now in the top row while the horizontal rule buttons were moved to the second row.
The underline and justify buttons are no longer available in WordPess 4.7. This is due to the fact that it causes confusion to the readers and some may interpret it as a link. However, the justify button was removed because it has uneven browser implementation and often makes text less readable.
In addition to these changes, tooltips that appear when hovering over a button will now display the keyboard shortcut. This makes them much more easier to read.
WordPress team is still not sure how these new changes will impact users. They will monitor user feedback and continue to make improvements accordingly. Since WordPress 4.7 is in beta 1 users are encouraged to test it and share feedback on the Alpha/Beta section of the support forums.