Add deprecation shim for __experimentalPluginPostExcerpt

[peterwilsoncc]

Description

wp.editPost.__experimentalPluginPostExcerpt was exposed as an accessible API in WordPress 6.5 and subsequently deprecated in #61188 with a no-op function.

This breaks backward compatibility for extenders using the API so the deprecation needs to be fixed to provide a shim for the recommended process for adding content to such panels.

Step-by-step reproduction instructions

POC plugin:

import { registerPlugin } from '@wordpress/plugins';
import { __experimentalPluginPostExcerpt } from '@wordpress/edit-post';

const PwccPostExcerpt = () => {
	return (
		<__experimentalPluginPostExcerpt className="pwcc-post-excerpt">
			Post Excerpt - Custom content
		</__experimentalPluginPostExcerpt>
	);
}

registerPlugin( 'pwcc-post-excerpt', { render: PwccPostExcerpt } );

1. Create and build the POC plugin above.
2. Checkout 4abdb39 or earlier
3. Create a new post and open the excerpt panel
4. At the bottom of the panel the text "Post Excerpt - Custom content" will display
5. Checkout trunk
6. Create a new post and open the excerpt panel
7. The plugin's text will no longer be displayed

Screenshots, screen recording, code snippet

WordPress trunk

Gutenberg 4abdb39

Gutenberg trunk

Environment info

• WordPress/WordPress master at 9a2a3dc5b3ed97ae7a894b65576227b033ec0e86
• Gutenberg trunk at 5b154e5

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[retrofox]

Hi, Peter. Thanks for creating this issue. 🙇‍♂️

The report is valid and logical.
However, the metrics indicate that external developers are not using the experimental plugin. Additionally, considering that the plugin API is not straightforward, well-defined, and diverges from the usual ways developers customize panels, I believe it is safe to deprecate and make it obsolete at this time.

[pdewouters]

@retrofox @peterwilsoncc thanks, so what's the recommended approach? I have some pending code changes using __experimentalPluginPostExcerptto add some text to the excerpt metabox, but if I understand correctly, this is deprecated in 6.6? So I'd rather avoid committing this and use a method that will still work in future.

[retrofox]

👋 Paul, do you think you could do it following these guidelines? https://developer.wordpress.org/block-editor/reference-guides/slotfills/plugin-document-setting-panel/#accessing-a-panel-programmatically
If so, you won't need the __experimentalPluginPostExcerpt. It will prevent you from using something that will soon be deprecated.

[pdewouters]

@retrofox so you mean remove it and add a custom one?

[retrofox]

You don't need the __experimentalPluginPostExcerpt to customize the post excerpt panel. For instance, we created a custom post excerpt with AI in the Jetpack plugin. You can probably do something similar.

[youknowriad]

I didn't realize that we actually nooped the component, I'm fine restoring the component as it was with a deprecation. We can noop in a future version.

[retrofox]

let's restore it then