function my_admin_notice() {
/*
* The class of admin notice should be "notice" plus any one of
* -"notice-error",
* -"notice-warning",
* -"notice-success"
* -"notice-info".
* Optionally use "is-dismissible" to apply a closing icon.
*/
echo '<div class="notice notice-info"><p>Custom notice text</p></div>';
}
add_action( 'admin_notices', 'my_admin_notice' );
The only thing is, these notices does not show up on New Post, Edit Post and similar where Gutenberg reins. There must be the solution, but I can’t find it right now.