Here is a plugin that does the same.
function get_posts_per_taxterm($taxonomy, $term_id ){
$posts = get_posts(
array(
'posts_per_page' => -1,
'post_type' => 'post',
'orderby' => 'menu_order',
'order' => 'ASC',
'tax_query' => array(
array(
'taxonomy' => $taxonomy,
'field' => 'term_id',
'terms' => $term_id,
'include_children' => false
)
)
)
);
// `first_post_id` is a global variable which contains the ID of the post which comes first inside TOC
// @toDo Find a better way
global $first_post_id;
if(!isset($first_post_id)){
if (!empty($posts)){
$first_post_id = $posts[0]->ID;
}
}
return $posts;
}
function get_array_of_posts_grouped_by_taxterms($terms, $parent_id = 0, $outputs = []) {
foreach ($terms as $term) {
if ($parent_id == $term->parent) {
$outputs['<a data-id = "'.$term->term_id.'" class = "elementor-kb-term-href stm-content" href="https://wordpress.stackexchange.com/questions/294817/.get_term_link($term->term_id).">'.$term->name.'</a>'] = get_array_of_posts_grouped_by_taxterms( $terms, $term->term_id);
$posts= get_posts_per_taxterm('category',$term->term_id);
foreach ($posts as $post){
$outputs['<a data-id = "'.$term->term_id.'" class = "elementor-kb-term-href stm-content" href="https://wordpress.stackexchange.com/questions/294817/.get_term_link($term->term_id).">'.$term->name.'</a>'][] = '<a class = "elementor-kb-post-href stm-content" href=".get_permalink($post->ID)." data-id="'.$post->ID.'">'.$post->post_title.'</a>';
}
}
}
return $outputs;
}