Brice Capobianco

Works well with the <a href="https://fr.wordpress.org/plugins/classic-editor/" rel="nofollow ugc">Classic Editor</a> plugin, <a href="https://fr.wordpress.org/plugins/advanced-custom-fields/" rel="nofollow ugc">Advanced Custom Fields</a> (Free & Pro) and is also compatible with the Gutenberg’s “Classic” bloc.
In WordPress 4.7, the core developper team decided to make various changes in the editor (TinyMce WYSIWYG), including removing the underline and justify buttons and rearranging some of the toolbar buttons. 
If you don’t want to change the way you edit your content and/or don’t want to confuse your customers with a new contribution interface, this very lightweight plugin will set the editor style back to its previous state (like in WordPress 4.6 and above versions).
You may change the Editor style from the Writing option page.
3 options are available for the Editor style:
<ul>
<li>Without underline & justify buttons</li>
<li>Default – Re-add underline & justify buttons</li>
<li>Re-add justify only</li>
</ul>
Please note, the previous option “Re-add underline & justify + rearrange” has been deprecated in 0.2 (sept. 2018) as it causes conflicts with the new Gutenberg editor. This option automatically switches to “Re-add underline & justify buttons” from now on.
<a href="https://www.b-website.com/re-add-text-underline-and-justify" title="Plugin documentation" rel="nofollow ugc">DOCUMENTATION</a>
<a href="https://www.b-website.com/category/plugins-en" title="More plugins by b*web" rel="nofollow ugc">CHECK OUT MY OTHER PLUGINS</a>
Please ask for help or report bugs if anything goes wrong. It is the best way to make the community benefit!

Re-add text underline and justify

This tiny plugin re-adds the Editor text underline & text justify buttons in the WYSIWYG removed in WordPress 4.7.0

Daniele Alessandra

IMPORTANT: This plugin is no longer updated since TinyMCE was replaced by Gutenberg since WordPress 5.0.
Please keep your sites up to date.

TinyMCE Recover

Restores two buttons removed in TinyMCE with WordPress 4.7: Underline and Justify.

Re-add text underline and justify vs TinyMCE Recover