I cannot find documentation or discussions on what is and how to use Drop-in plugins.
The question was raised in this Q&A, Populate content on install.
wp-content/install.php file that redefines the pluggable function
wp_install_defaults a Drop-in plugin or not?
My research results
The following list is from hakre's article (which seems to be the origin of all other copies in the interwebs).
File Type of Plugin Loaded Context advanced-cache.php Advanced caching plugin. on WP_CACHE value Single db.php Custom database class always Single db-error.php Custom database error message on error Single install.php Custom install script on install Single maintenance.php Custom maintenance message on maintenance Single object-cache.php External object cache always Single sunrise.php Executed before Multisite is loaded on SUNRISE value Multi blog-deleted.php Custom blog deleted message on deleted blog Multi blog-inactive.php Custom blog inactive message on inactive blog Multi blog-suspended.php Custom blog suspended message on archived or spammed blog Multi Source: Data taken from global function _get_dropins() in wp-admin/includes/plugin.php.
In the Codex a Drop-in is defined as:
The concrete PHP-file
At first, there's nothing on [wp-hackers] list. And searching for "drop-in + wordcamp" I've only found this of relevance my emphasis:
One of the great strengths of WordPress is its plugin system. In the upcoming 2.1 release, there is very little you can’t do as a drop-in plugin. This makes it easy to use WordPress as a CMS and keep your customizations separate from the main codeline; which in turn simplifies upgrades and maintenance.