Category Post list toggle

category-posts.php

function category_posts_shortcode() {

// Get all categories with posts
$categories = get_categories(array(
    'hide_empty' => 1
));

$output = '<ul class="category-list">';

// Loop through each category
foreach ($categories as $category) {

    $output .= '<li class="category-item">';
    $output .= '<input type="checkbox" id="' . $category->slug . '" class="category-toggle">';
    $output .= '<label class="category-title" for="' . $category->slug . '">' . $category->name . '</label>';
    $output .= '<ul class="post-list">';

    // Get all posts in the category
    $posts = get_posts(array(
        'category' => $category->term_id,
        'posts_per_page' => -1
    ));

    // Loop through each post and add it to the output
    foreach ($posts as $post) {
        $output .= '<li class="post-item"><a href="' . get_permalink($post->ID) . '">' . $post->post_title . '</a></li>';
    }

    $output .= '</ul>'; // .post-list
    $output .= '</li>'; // .category-item

}

$output .= '</ul>'; // .category-list

return $output;

}
add_shortcode('category_posts', 'category_posts_shortcode');

Some CSS to style and make toggles of labels:

.category-toggle {
  display: none;
}

.category-title {
  cursor: pointer;
  display: flex;
  align-items: center;
  position: relative;
  gap: 1em;
}

.category-title::before {
  content: "\25B8";

}

.category-toggle:checked+.category-title::before {
  transform: rotate(90deg);
}

.category-list,
.category-list li {
  list-style-type: none;
  margin-left: 0;
}

.post-list {
  display: none;
  padding: 10px 0;
}

.category-toggle:checked+.category-title+.post-list {
  display: block;
}

To use - add: [category_posts] shortcode to where you want it displayed.