Jenst

Add a short note about the latest changes in the current post or page.
WP Update message adds a metabox in admin post or page section. If you updated a post or page with important information you can write a note about it here.
There are a few optional settings, but the plugin works fine even without changing anything.
<h4>Settings</h4>
You can choose a style theme and more.
<ul>
<li>Published date format (if published date should be included)</li>
<li>Updated date format (if updated date should be included)</li>
<li>Position</li>
<li>HTML</li>
<li>New row replacement</li>
<li>Style theme</li>
</ul>

WP Update Message

Add a short note about the latest changes in the current post or page.

Dreb Bitanghol

This field will provide you more options and flexibility on how you would like to present an instruction or message to your client’s (or site in general) admin interface.
Features:
<ul>
<li>Support PHP.</li>
<li>Option to hide label when field is displayed.</li>
</ul>
Compatible with Advanced Custom Fields:
<ul>
<li>4 (tested up to 4.4.6)</li>
<li>5 (PRO, tested up to 5.3.7)</li>
</ul>
PS: You need <a href="https://wordpress.org/plugins/advanced-custom-fields/" rel="ugc">Advanced Custom Fields</a> installed to use this.
PPS: Even more awesome if you have <a href="http://www.advancedcustomfields.com/pro/" rel="nofollow ugc">Advanced Custom Fields PRO</a> installed.
Important note: Use this with utmost care.

ACF Enhanced Message Field

Exactly what I needed

Exactly what I needed. You could improve the plugin by making the textarea field look more like an IDE, with system fonts, syntax highlights and so on.

Perfect

Awesome!!

Adds an enhanced version of the default Message field to accept PHP and certainly no wpauto().

WP Update Message vs ACF Enhanced Message Field