show a custom message/notice after a NEW WordPress post has been published.

WP-custom-notice-after-new-post.php

<?php
add_action('admin_notices', function(){
    $screen = get_current_screen();     

    $editPostScreen = ($screen->parent_base == 'edit' && $screen->post_type == 'post') ? true : false;
    $justPublished = (isset($_GET['message']) && $_GET['message'] == '6') ? true : false;

    if ($editPostScreen && $justPublished) {

        $class = 'notice notice-info';
        $message = __( 'Just published a post custom text', 'myplugin_textdomain' );
        printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) ); 

    }
});

// ($justPublished relies on $_GET['message']. This URL query variable is not visible in admin address 
// bar as Wordpress actually removes it from the URL with Javascript! Took me a long while to figure 
// that out, so although you can't see it, it actually exists!).

add_filter( 'post_updated_messages', function($messages){
    $messages['post'][6] = __( 'Just published a post custom text', 'myplugin_textdomain' );
    return $messages;
}, 10, 1 );