As a WordPress developer, it would be helpful to have a reference guide to:

• AllMost of the ways a theme, plugin or third-party code can integrate with the WordPress classic editor.
• Recommendation to achieve equivalent functionality with Gutenberg.

I've started working on such a document, WordPress Editor Integration Points, and can grant contributor access to whomever is interested. Alternatively, please leave comments on this issue with existing classic editor integrations that I don't yet have documented.

Once the document is more substantive, we can put it in a public place.

Related:

• Optimize for the majority of WordPress users able to use Gutenberg on day one Optimize for the majority of WordPress users able to use Gutenberg on day one #4087
• Quantify plugin and theme compatibility with Gutenberg Quantify plugin and theme compatibility with Gutenberg #4072
• Document known changes in editor behavior Document known changes in editor behavior #4186