WordPress 4.7 is set to feature some noteworthy changes to the editor.
For easier access and to encourage proper use of the HTML elements, some of the toolbar buttons have been rearranged in 4.7. The headings drop down can be found on the top row, whereas the strike-through and horizontal rule button will be moved down.
Often, underlining is considered a bad practice as it confuses the readers, and also it does not insert a semantic element. Associating this issue with bad accessibility, the underline button has now been removed from the bottom row. The bottom row will also be sans the justify buttons. Justifying has uneven browser implementation, and in many cases compromises with the readability. Keyboard shortcuts for both will keep working though.
For more information, you can see #27159 that also has links to the discussions in Slack.
Labels for keyboard shortcuts have been added to the tooltips for buttons and inside drop downs to make them easier to discover.