// Get some posts 
$posts = Timber::get_posts([ "post_type" => "projects", "category_name" => "featured", "posts_per_page" => "-1" ]);

// Make an array to hold the query's active taxonomies
$query_taxonomies = [];

// Loop through all the posts in the query
foreach ($posts as $post) {
	// Then loop through their terms
	foreach ($post->terms() as $term) {
		// Add a key to the array of taxonomies if it doesn't have one
		if (!array_key_exists($term->taxonomy, $query_taxonomies)) {
			$query_taxonomies[$term->taxonomy] = [];
		}

		// Add the term's ID to the corresponding taxonomy array if it isn't already included
		if (!in_array($term->id, $query_taxonomies[$term->taxonomy])) {
			$query_taxonomies[$term->taxonomy][] = $term->id;
		}
	}
}

// Loop through the query's active taxonomies, use the array of IDs to get an array of term objects, and add those to the Timber context
foreach ($query_taxonomies as $filter_key => $filter_term_ids) {
	if (!empty($filter_term_ids)) {
		$terms = Timber::get_terms([
			"taxonomy" => $filter_key,
			"include" => $filter_term_ids,
		]);
		$context["filters"][$filter_key] = $terms;
	}
}