When you set up unrouted@domain.com in the email forwarders, I will take any emails sent to non-existant accounts at the domain and route them to the address provided.

I found today, when I set up a forwarder for unrouted@subdomain.domain.com, that it did not collect the emails and instead it responded to the sender with a bounce message for an unrouteable address. Is this a limitation with subdomains?

You can set up default email address for subdomains.
There is an ability to forward all unrouted mail to main domain account or some email address specific for every subdomain