related_post_loop_item

Display custom HTML under item class.

add_action('related_post_loop_item','related_post_loop_item_20200126', 0);

function related_post_loop_item_20200126($atts) {

    $loop_post_id = isset($atts['loop_post_id']) ? (int) $atts['loop_post_id'] : get_the_ID();

    ?>
    Custom HTML for item
    <?php

}

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Remove hook

add_action('related_post_loop_item','related_post_loop_item_remove_20200126', 0);

function related_post_loop_item_remove_20200126($loop_post_id) {

    remove_action('related_post_loop_item','related_post_loop_item');

}