Load a single post page based on the category (can be adapted to taxonomy)

functions.php

<?php
/**
 * Load Single Page by Category
 */

add_filter('single_template', function ($template) {
    foreach ((array) get_the_category() as $cat) {
        $templateFile = STYLESHEETPATH . "/single-category-{$cat->slug}.php";

        if (file_exists($templateFile)) {
            return $templateFile;
        }

        if ($cat->parent) {
            $cat = get_the_category_by_ID($cat->parent);
            $templateFile = STYLESHEETPATH . "/single-category-{$cat->slug}.php";

            if (file_exists($templateFile)) {
                return $templateFile;
            }
        }
    }
    return $template;
});

/**
* Sage 9 Version
**/
add_filter('single_template', function ($template) {
    foreach ((array) get_the_category() as $cat) {
        $templateFile = STYLESHEETPATH . "/views/single-category-{$cat->slug}.blade.php";

        if (file_exists($templateFile)) {
            return $templateFile;
        }

        if ($cat->parent) {
            $cat = get_the_category_by_ID($cat->parent);
            $templateFile = STYLESHEETPATH . "/views/single-category-{$cat->slug}.blade.php";

            if (file_exists($templateFile)) {
                return $templateFile;
            }
        }
    }
    return $template;
});