shprung

Less than 40 lines of php code so it will not slow your site and if all you 
need is to add an rss feed, give it a try.
You can even setup the output template – something many other plugin don’t 
offer.
<h3>A brief Markdown Example</h3>
<pre><code>short code examples:

[rss url="http://www.usacycling.org/rss/headlines.rss"]


[rss url="http://www.usacycling.org/rss/headlines.rss" no="20" target="_self"]

You can change the template used via wp-admin/options.php , look for
'rss_shortcode_template'
Template fields documentation:
 _TITLE_ : the rss feed title line
 _LINK_ : the link to the complete store, aka permalink
 _MSG_ : the short message/description; the first few 
 lines of the post/story
 _DATE_ : post date & time
 _TARGET_ : the link target
</code></pre>

RSS shortcode w/template

lets you embed rss feed in any WordPress content (e.g., pages, posts, custom post types) using Shortcode.

Marios Alexandrou

Modifies RSS feeds so that they include pages and not just posts. Deactivating the plugin restores RSS feeds to their default state.
Including pages in your feed is particularly useful if you’re using WordPress as a CMS where pages represent a good portion of your content.
The <a href="https://infolific.com/technology/software-worth-using/include-pages-in-wordpress-rss-feeds/#pro-version" rel="nofollow ugc">pro version</a> (a lifetime license is less than $15) also allows you to:
<ol>
<li>Specify that ONLY pages are included in the feeds i.e. no posts.</li>
<li>Exclude/include posts and pages by ID.</li>
<li>Add custom post types such as WooCommerce products or Avada portfolios.</li>
</ol>

RSS Includes Pages

Some people say this plugin is useless, but they don't seem to be reading the description. If you want pages to be added to your existing feeds, this plugin will do that. No settings or configuration. Just activate and go.
Some additional features are in the pro version, but 30,000+ users find the free version useful enough to keep it active. You can't argue with the numbers! :-)



Adds Pages To Feeds Just Like It Says

Completely useless plugin. Only includes posts and pages into the RSS feed. For custom post types you have to pay $10. If you plan to make money with your plugins, please say so in the description so people don't waste their time.



Completely useless

Modifies RSS feeds so that they include pages and not just posts.

RSS shortcode w/template vs RSS Includes Pages