Site Announcement Block Managed in Settings

add_action( 'admin_init', function () {
    register_setting( 'general', 'wpide_announcement', array(
        'type'              => 'string',
        'sanitize_callback' => 'wp_kses_post',
        'default'           => '',
    ) );
    add_settings_field( 'wpide_announcement', '<label for="wpide_announcement">Announcement</label>', function () {
        printf(
            '<textarea id="wpide_announcement" name="wpide_announcement" rows="3" class="large-text">%s</textarea><p class="description">Shown wherever the Announcement block is placed. Leave empty to hide it.</p>',
            esc_textarea( (string) get_option( 'wpide_announcement', '' ) )
        );
    }, 'general' );
} );

add_action( 'init', function () {
    wp_register_script( 'wpide-block-announcement', false, array( 'wp-blocks', 'wp-element', 'wp-block-editor', 'wp-server-side-render' ), '1.0', true );
    wp_add_inline_script( 'wpide-block-announcement', <<<'JS'
( function ( wp ) {
    var el = wp.element.createElement;
    wp.blocks.registerBlockType( 'wpide/announcement', {
        apiVersion: 3,
        title: 'Announcement',
        icon: 'megaphone',
        category: 'widgets',
        edit: function () {
            return el( 'div', wp.blockEditor.useBlockProps(),
                el( wp.serverSideRender, { block: 'wpide/announcement', EmptyResponsePlaceholder: function () { return el( 'p', null, 'The announcement is empty. Set it in Settings > General.' ); } } )
            );
        },
        save: function () { return null; }
    } );
} )( window.wp );
JS
    );

    register_block_type( 'wpide/announcement', array(
        'api_version'           => 3,
        'title'                 => 'Announcement',
        'category'              => 'widgets',
        'editor_script_handles' => array( 'wpide-block-announcement' ),
        'render_callback'       => function () {
            $message = trim( (string) get_option( 'wpide_announcement', '' ) );
            if ( '' === $message ) {
                return '';
            }
            return '<div ' . get_block_wrapper_attributes( array( 'class' => 'wpide-announcement', 'role' => 'note' ) ) . '>' . wpautop( wp_kses_post( $message ) ) . '</div>';
        },
    ) );
} );