RaymondDesign

When you want to place a small text rule, an advertisement or something else at the top or bottom of your post or page, it is possible to edit your theme. But you lose all your changes when you update you theme. 
With Post & Page Notes, shortly PPNotes you can put your own text or image on a post or page, without editing your theme. So, when you update your theme or something else the Post & Page Notes are kept.
When you have installed this plugin, you have two forms, one for the header text and one for the footer text. For every form (header and footer), you can set where you want to show the text. You can choose from a list: ‘Post’, ‘Page’ and ‘Post & Page’. If you choose ‘Post’ the header or footer is visible at every single post, if you choose ‘Page’ the header or footer is visible on every page (except the pages with single posts and the archives). If you choose ‘Page & Post’ the header or footer is visible on both single posts and pages.
In the text fields, you can type plain text or (x)HTML, so it’s possible to place (nice styled) div’s or images at the top of your posts or pages.

Post & Page Notes

Post & Page Notes, shortly PPNotes is a plugin to simply at a header and footer to your posts and pages, without editing your theme.

Webliberty

Use the Simple Note plugin to write informative and warning notes, notes about errors or success, as well as quotes in colored blocks.
<h3>How to use</h3>
Search beautiful colored blocks in Common blocks category and insert into the content.
<h3>Translations</h3>
You can translate Simple Note on <a href="https://translate.wordpress.org/projects/wp-plugins/simple-note" rel="nofollow ugc">translate.wordpress.org</a>.

Simple Note

The plugin allows you to create colored text notes in the new block editor.

Post & Page Notes vs Simple Note