Should not also $current_categories_description be removed from
$zco_notifier->notify('NOTIFY_HEADER_END_INDEX_MAIN_TEMPLATE_VARS', NULL, $current_categories_description);
since it has no effect if modified?

No. We want to avoid not-upwardly-compatible changes to notifers.

Understood as a general rule, but given a previous comment to make the function of a notifier self-explanatory, leaving this here implies it can be changed, when it can't, so it is misleading.

It can be changed. The change just won't show up in the template. It could be used somewhere else, who knows.

I wonder if the placement of the original notifier is the real problem here.
Yes, our standard practice is to leave most final notifier calls at the bottom of these header_php and main_template_vars files .... but in this sort of case where tpl_page_body is called manually (as is the case in all main_template_vars files), there's little point in the notifier hook coming after it.
I haven't looked at the overall code-base to see how drastic this sort of pattern correction might be.

Let's not overcorrect a non-problem based on a speculation. The original issue is resolved. Unless there's an actual problem with the last notifier, this should be sufficient.