A quick grep revealed at least a couple callers still passing the `addon` parameter:
apps/api/templates/api/includes/addon.xml:    size="{{ file.size }}">{{ file.get_url_path('api', addon) }}</install>
apps/api/templates/api/includes/addon.xml:      {{ file.get_url_path('api', addon) }}
apps/addons/buttons.py:            url = file.get_url_path(self.src, self.addon)

I'm worried there might be more, but this could be tricky to find...