[Per Slack request] Illustrating the use of WordPress data selectors in conjunction with React hooks to react to post saving within a custom sidebar in Gutenberg. Move advanced entity-centric selectors may be used from the 'core' store instead.

my-test.php

<?php
/*
Plugin Name: Test
*/

function my_test_enqueue_scripts() {
	wp_enqueue_style(
		'my-test-style',
		plugins_url( 'style.css', __FILE__ )
	);
	wp_enqueue_script(
		'my-test-plugin',
		plugins_url( 'plugin.js', __FILE__ ),
		array( 'wp-element', 'wp-components', 'wp-editor', 'wp-edit-post', 'wp-plugins' )
	);
}
add_action( 'enqueue_block_editor_assets', 'my_test_enqueue_scripts' );

plugin.js

( function( wp ) {
	const { createElement: el, useEffect, useRef } = wp.element;
	const { Panel, PanelBody } = wp.components;
	const { registerPlugin } = wp.plugins;
	const { PluginSidebar } = wp.editPost;
	const { useSelect } = wp.data;
 
    registerPlugin( 'my-plugin-sidebar', {
        render() {
            return el( PluginSidebar,
                {
                    name: 'my-plugin-sidebar',
                    icon: 'admin-post',
                    title: 'Save status',
                },
				el( SaveStatus )
            );
        },
    } );

	function SaveStatus() {
		const hasJustSaved = useDidSave();
		useEffect( () => {
			if ( hasJustSaved ) console.log( 'Post saved!' );
		}, [ hasJustSaved ] );

		return el( Panel, {},
			el( PanelBody, {},
				'Have we just saved? ',
				el( 'span', { className: hasJustSaved ? 'my-test-has-just-saved' : '' },
					hasJustSaved ? 'Yes.' : 'No.'
				)
			)
		);
	}

	function useDidSave() {
		const wasSaving = useRef( false );
		const { isDirty, isSaving } = useSelect( ( select ) => {
			const { isEditedPostDirty, isSavingPost } = select( 'core/editor' );
			return {
				isDirty: isEditedPostDirty(),
				isSaving: isSavingPost(),
			};
		} );

		const hasJustSaved = wasSaving.current && ! isSaving;
		if ( isDirty ) wasSaving.current = isSaving;
		return hasJustSaved;
	}


} )( window.wp );

style.css

@keyframes my-test-glow {
	from {
		text-shadow: 0 0 10px gold;
	}

	to {
		text-shadow: 0 0 0 gold;
	}
}

.my-test-has-just-saved {
	animation: 1s my-test-glow;
}