Gregg

This plugin allows you to change the footer text on the bottom left of the admin page.
Once activated, navigate to the Settings -> nuAdmin Footer and change the Footer text/HTML. You can use html to link to your website, your help desk, your email or anything you would like. If you find the plugin to be useful and would like to donate, you can do so in the Settings.
Here’s a link to <a href="http://www.tripleginteractive.com/blog/wordpress/custom-admin-footer/" title="free wordpress plugin: custom admin footer" rel="nofollow ugc">the blog post on this topic</a> for more information. Future version’s will have much more capabilities.

nuAdmin Footer

This plugin allows you to change the footer text on the bottom left of the admin page.

SOGO

Tested with Gutenberg
Create a simple way to add javascript code to individual page post or custom post type header and footer, 
for example: add conversion code to thank you pages 
add google re-marketing code to individual pages 
and much more… 
added in version 1.3 the option to add script \ style to all pages not only individual pages, 
 this will allow you to add Google re-marketing code to the entire site or Google Analytics 
 to use it goto “settings” – “Header Footer Settings”
New Features (version 3.0): 
Added option for terms / category pages
New Features (version 2.3): 
 Added support for WooCommerce shop page.
<ol>
<li>
we support now exclude individual pages form printing the header and footer scripts, this is supported by 2 checkboxes in each page\post 
buy check it the script will not be display on this page
</li>
<li>
we added support to limit the script only for certain post type \ page
</li>
</ol>
if you like it \ use it – please rate us.
usage: 
You need to paste the code with the script tag, for example: 
 you js code

SOGO Add Script to Individual Pages Header Footer

Simple plugin to add script to header and footer for individual pages & posts

nuAdmin Footer vs SOGO Add Script to Individual Pages Header Footer