Source

File: wp-includes/class-wp-customize-manager.php

protected function dismiss_user_auto_draft_changesets() {
		$changeset_autodraft_posts = $this->get_changeset_posts(
			array(
				'post_status'               => 'auto-draft',
				'exclude_restore_dismissed' => true,
				'posts_per_page'            => -1,
			)
		);
		$dismissed                 = 0;
		foreach ( $changeset_autodraft_posts as $autosave_autodraft_post ) {
			if ( $autosave_autodraft_post->ID === $this->changeset_post_id() ) {
				continue;
			}
			if ( update_post_meta( $autosave_autodraft_post->ID, '_customize_restore_dismissed', true ) ) {
				$dismissed++;
			}
		}
		return $dismissed;
	}