Add thumbnail field to WordPress category via REST API

get_thumbnail_to_category_rest.php

<?php
add_action('rest_api_init', 'register_my_rest_field');
/**
 * It depends on term_meta key _thumbnail_id using the metadata API
 * for terms. Only plugin I know uses this is Taxonomy Thumbnail:
 * https://wordpress.org/plugins/sf-taxonomy-thumbnail/ instead of
 * storing metadata for terms in wp_options or custom tables
 */
function register_my_rest_field()
{
    register_rest_field('category', 'thumbnail', [
        'get_callback' => 'dummy_plug',
        'update_callback' => null,
        'schema' => [
            'description' => 'Holds the thumbnail pic URL',
            'type' => 'string',
            'format' => 'url',
            'context' => ['view'],
            'readonly' => true,
        ]
    ]);
}
function dummy_plug($object, $field_name, $request) {
    $attachment_id = get_term_meta($object['id'], '_thumbnail_id', true);
    if(!$attachment_id) {
        return;
    }
    $url = wp_get_attachment_url($attachment_id);
    return $url;
}