<?php

add_action('rest_api_init', 'endlessPosts');

function endlessPosts()
{
 register_rest_route('api-posts/v1', "/endlessPosts", array(
    'methods' => WP_REST_Server::READABLE,
    'callback' => 'getEndlessPostsResults'
 ));
}

function getEndlessPostsResults($data, $paged = 1)
{
    global $wpdb;

    $link = $data['permalink'];
    $id_from_permalink = explode('/', $link);
    $id_from_permalink = explode('-', $id_from_permalink[3]);

    $id = $id_from_permalink[0];

    $category = get_the_category($id);

    $result = array();

    $args = array(
        'category__in' => $category[0]->term_id,
        'posts_per_page' => 10,
        'paged' => $paged
    );

    $posts = new WP_Query($args);

    while ( $posts->have_posts() ) {
        $posts->the_post();

        $terms = get_the_category();

        $result[] = array(
            'permalink' => get_the_permalink(),
            'terms'     => $terms[0]->slug,
        );
    }

    return $result;
}

//"http://pamtest.ru/702-oformlenie-osago-na-a-b-c-d-otkryto-vsem-agenta/"

//"http://pamtest.ru/294-6-j-urok-kak-strahovomu-agentu-zadavat-voprosy-i-rabotat-s-vozrazheniyami/"