Per #31650, there’s been a change to the way headers are processed.
Previously we used H2 in order to create the header The header of your site is typically the first thing people will experience. The masthead or header art located across the top of your page is part of the look and feel of your website. It can influence a visitor’s opinion about your content and you/ your organization’s brand. It may also look different on different screen sizes. at the top of our pages like this:
<div class="wrap">
<h2><?php _e("My Cool Plugin", 'my-plugin'); ?></h2>
<p>All my cool dashboard things</p>
</div>
This was done because, prior to the Toolbar, WordPress put in an H1 header itself. With the change to the Toolbar quite a while ago, that was removed and it created a new problem for accessibility Accessibility (commonly shortened to a11y) refers to the design of products, devices, services, or environments for people with disabilities. The concept of accessible design ensures both “direct access” (i.e. unassisted) and “indirect access” meaning compatibility with a person’s assistive technology (for example, computer screen readers). (https://en.wikipedia.org/wiki/Accessibility). In order to fix that issue the H2s were changed to H1s in core Core is the set of software required to run WordPress. The Core Development Team builds WordPress..
This will not break your plugins.
There is no visual difference, the styles stayed the same. Only the underlying markup has been changed.
If you want to fix your plugins, just change that H2 to H1 and call it a day. Screenreaders will thank you.