Jamie Dąbrowiecki

Headingo is a lightweight utility plugin that allows administrators to control which heading levels are available in the editor toolbar. This tool only works for websites using the Classic Editor.
These settings are applied to all users with editing permissions.
The configurable heading levels are:
<ul>
<li>Heading 1</li>
<li>Heading 2</li>
<li>Heading 3</li>
<li>Heading 4</li>
<li>Heading 5</li>
<li>Heading 6</li>
<li>Preformatted</li>
</ul>
This is especially useful for removing the Heading 1 (H1) option to prevent editors from mistakenly selecting it, as well as removing heading levels which are not required on your website.
This plugin is intended for websites that rely on the Classic Editor editing experience. It does not attempt to modify the Block Editor (Gutenberg), where heading availability is governed by editor and theme configuration (theme.json) by design. The Classic Editor plugin is required for this plugin to work (or a theme that allows you to switch to the Classic Editor without the plugin).
<h3>Classic Editor</h3>
The Classic Editor (TinyMCE) is fully supported. Selected heading levels and Preformatted can be hidden from the formatting dropdown while remaining available via keyboard shortcuts or pasted HTML. The Paragraph option is always available and cannot be toggled.
<h3>Block Editor (Gutenberg)</h3>
This plugin does not control heading levels in the Block Editor. Gutenberg does not currently provide a reliable API for plugins to override available heading levels, as these are controlled by editor and theme settings.

Headingo

Choose which heading levels are visible in the Classic Editor toolbar.

Stefan Matei

If your subheadings appear on archive pages, they need to start from H3 (because H2 tags are used by the post titles on archive pages).
This plugin dynamically corrects subheadings before they are displayed on your site — for SEO and semantic markup purposes.
You will be able to select one of the following two options (the first one being the default):
<ul>
<li>
My subheadings start from the H2 level: turn them down a level on archive pages. 
H2’s become H3’s, H3’s become H4’s, H4’s become H5’s, H5’s become H6’s and H6’s become paragraphs
</li>
<li>
My subheadings start from the H3 level: turn them up a level on single posts or pages. 
H3’s become H2’s, H4’s become H3’s, H5’s become H4’s and H6’s become H5’s
</li>
</ul>
This plugin does not make any changes to your database: the subheadings will only be displayed differently on the front-end site. 
If you disable the plugin or decide to uninstall it everything will be back to normal.
Visit the <a href="http://www.vileworks.com/correct-my-headings" title="Correct My Headings" rel="nofollow ugc">plugin page</a> on VileWorks.com.

Correct My Headings

If your subheadings appear on archive pages, they need to start from H3 (because H2 tags are used by the post titles on archive pages).

Headingo vs Correct My Headings