Create an array of posts group by alphabet with WP_Query.
Use below to create the list of posts array with the group by alphabet.
Note: I have used
post type. You can change this post type with your own. Also, you can modify the WP_Query parameters.
Now we can show the posts with the group by alphabet with a shortcode.
In below code we have created a shortcode
prefix_alphabetic_posts to show the result in the post.
NOTE: Below short code is created only for demo purpose. So, Added inline CSS. You need to use wp_enqueue_scripts to enqueue your scripts.