WordPress at a glance
action is not described

admin_footer-widgets.php action-hook . WP 1.0

This action is documented in wp-admin/admin-footer.php

Usage

add_action( 'admin_footer-widgets.php', 'action_function_name_1566' );
function action_function_name_1566(){
	// action...
}

Где вызывается хук

WP_Customize_Widgets::print_footer_scripts()
admin_footer-widgets.php
wp-includes/class-wp-customize-widgets.php 867
do_action( 'admin_footer-widgets.php' ); // phpcs:ignore WordPress.NamingConventions.ValidHookName.UseUnderscores

Где используется хук в ядре WP

wp-includes/widgets/class-wp-widget-custom-html.php 79
add_action( 'admin_footer-widgets.php', array( 'WP_Widget_Custom_HTML', 'render_control_template_scripts' ) );
wp-includes/widgets/class-wp-widget-media.php 119
add_action( 'admin_footer-widgets.php', array( $this, 'render_control_template_scripts' ) );
wp-includes/widgets/class-wp-widget-text.php 70
add_action( 'admin_footer-widgets.php', array( 'WP_Widget_Text', 'render_control_template_scripts' ) );