Adding an option to post editor to show a site disclaimer or message

The thing you're talking about is called a custom meta box. They can be used to save post specific settings and data. You can create your own meta boxes with the help of add_meta_box() function.

Here's a quick example. This would go to your (child) theme's functions.php file. Alternatively if you want to make the code theme independent, then create a custom plugin for the code below. The example works in both Classic and Block editors.

// Register metabox only for "post" post type
add_action('add_meta_boxes_post', 'my_prefix_register_my_custom_metabox', 10, 1);
function my_prefix_register_my_custom_metabox( $post ) {
    add_meta_box(
        'my-metabox', // id
        esc_html__( 'My Meta Box', 'textdomain' ), // title
        'my_prefix_render_my_custom_metabox', // callback
        'post', // screen - typically post type
        'side', // context - normal, side, advanced
        'high', // priority - high, low
        array() // optional callback args
    );
}

// Callback function that add_meta_box uses to render the contents of the metabox
function my_prefix_render_my_custom_metabox( $post, $callback_args ) {
    $checked = get_post_meta( $post->ID, '_my_checkbox_value', true );
    wp_nonce_field( 'my_custom_metabox_nonce_action_' . $post->ID, 'my_custom_metabox_nonce' );
    ?>
    <label>
        <input type="checkbox" name="_my_checkbox_value" value="1" <?php checked( $checked, 1); ?>>
        <span><?php esc_html_e( 'Display disclaimer', 'textdomain' ); ?></span>
    </label>
    <?php
}

// Callback for saving the metabox form fields when the post is saved
// Executes only when saving "post" post type
add_action( 'save_post_post', 'my_prefix_save_my_custom_metabox', 10, 3 );
function my_prefix_save_my_custom_metabox( $post_id, $post, $update ) {
    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }
    if (
        wp_is_post_autosave( $post_id ) ||
        wp_is_post_revision( $post_id )
    ) {
        return;
    }
    if (
        empty( $_POST['my_custom_metabox_nonce'] ) ||
        ! wp_verify_nonce( $_POST['my_custom_metabox_nonce'], 'my_custom_metabox_nonce_action_' . $post_id )
    ) {
        return;
    }
    if ( isset( $_POST['_my_checkbox_value'] ) ) {
        update_post_meta( $post_id, '_my_checkbox_value', 1 );
    } else {
        delete_post_meta( $post_id, '_my_checkbox_value', 1 );
    }
}

You can also add to the same file a helper function which takes care of displaying the disclaimer message.

function my_prefix_display_disclaimer() {
    if (
        is_single() &&
        get_post_meta( get_the_ID(), '_my_checkbox_value', true )
    ) {
        ?>
        <aside class="disclaimer">
            <p>This is a disclaimer.</p>
        </aside>
        <?php
    }
}

You can then either use the helper function directly in some (child) theme template file.

<?php my_prefix_display_disclaimer(); ?>

Or hook it into an action hook that your theme might provide.

// in functions.php or custom plugin file
add_action( 'some_theme_template_action_hook', 'my_prefix_display_disclaimer' );