list snippets without category terms
wp eval '
$args = [
"post_type" => "snippet",
"posts_per_page" => -1,
"tax_query" => [
[
"taxonomy" => "snippet_category",
"operator" => "NOT EXISTS",
],
],
];
$posts = get_posts($args);
if (empty($posts)) {
echo "No snippet posts without a category found.\n";
} else {
$ids = wp_list_pluck($posts, "ID");
echo implode(" ", $ids) . "\n";
}'