Index: src/wp-admin/about.php
===================================================================
--- src/wp-admin/about.php	(revision 43913)
+++ src/wp-admin/about.php	(working copy)
@@ -24,9 +24,25 @@
 		<h1><?php printf( __( 'Welcome to WordPress&nbsp;%s' ), $display_version ); ?></h1>
 
 		<p class="about-text"><?php printf( __( 'Thank you for updating to the latest version! WordPress %s introduces a robust new content creation experience.' ), $display_version ); ?></p>
-		<p class="about-text">
-			<a href="#classic-editor"><?php _e( 'Learn how to keep using the old editor.' ); ?></a>
-		</p>
+
+		<?php if (
+			// Was the Gutenberg plugin installed before upgrading to 5.0.x?
+			get_option( 'upgrade_500_was_gutenberg_active' ) == '1'  &&
+			current_user_can( 'activate_plugins' ) &&
+			// Has it not been reactivated since?
+			is_plugin_inactive( 'gutenberg/gutenberg.php' ) &&
+			// Is it still installed?
+			file_exists( WP_PLUGIN_DIR . '/gutenberg/gutenberg.php' )
+		) : ?>
+			<div class="about-text" style="font-style:italic;">
+				<?php printf( __( 'The Gutenberg plugin has been deactivated, as the features are now included in WordPress %1$s by default. If you\'d like to continue to test the upcoming changes in the WordPress editing experience, please %2$sreactivate the Gutenberg plugin%3$s.' ), $display_version, '<a href="' . esc_url( self_admin_url( 'plugins.php?s=gutenberg&plugin_status=all' ) ) . '">', '</a>' ); ?>
+			</div>
+		<?php else : ?>
+			<p class="about-text">
+				<a href="#classic-editor"><?php _e( 'Learn how to keep using the old editor.' ); ?></a>
+			</p>
+		<?php endif; ?>
+
 		<div class="wp-badge"><?php printf( __( 'Version %s' ), $display_version ); ?></div>
 
 		<h2 class="nav-tab-wrapper wp-clearfix">
Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(revision 43913)
+++ src/wp-admin/includes/upgrade.php	(working copy)
@@ -1819,11 +1819,18 @@
  */
 function upgrade_500() {
 	global $wp_current_db_version;
	if ( $wp_current_db_version < 43764 ) {
 		// Allow bypassing Gutenberg plugin deactivation.
 		if ( defined( 'GUTENBERG_USE_PLUGIN' ) && GUTENBERG_USE_PLUGIN ) {
 			return;
 		}
+
+		$was_active = is_plugin_active( 'gutenberg/gutenberg.php' );
+		if ( $was_active ) {
+			// FIXME: Leave until 501 or 510 to clean up.
+			update_site_option( 'upgrade_500_was_gutenberg_active', '1' );
+		}
+
 		deactivate_plugins( array( 'gutenberg/gutenberg.php' ), true );
 	}
 }