Karrde

Includes a file’s contents in the widget. Quick and dirty, all this does is use the PHP include function to include the specified file in the widget. There is NO checking done whatsoever on the file, so use it at your own risk. I wrote this because I couldn’t find a widget to do this ? plugins exist for using include in posts and pages, but not in widgets.

Include Widget

Includes a file's contents in the widget.

kashanshah

This plugin enables users to add the same content to multiple posts and pages using a shortcode. It functions similarly to PHP’s ‘include’ function.
Users can include HTML, CSS, and JavaScript in their content with a simple shortcode. 
For example, if my brand’s tagline, “The Best Of Its Kind,” needs to be displayed on various pages and posts, I would add it to the Included Content section. The plugin will generate a shortcode for this content, which I can then place wherever I want the tagline to appear.
Additionally, CSS and JavaScript can be added alongside the HTML content, ensuring they are included wherever the shortcode is used.
In the future, if I change my tagline, I only need to update it in the Included Content section, and it will be updated everywhere the shortcode has been used.

Include Content By Shortcode

I’ve been using it on multiple sites. It helps me managing website a lot. A lot of sections are used in multiple pages. Using these sections through shortcode helps me synchronisation among the same sections throughout the website. 

very helpful

This plugin allows you to include content from a custom post type by using a shortcode. You can also add custom CSS and JS to the included content.

Include Widget vs Include Content By Shortcode