Help in understanding Hinted Handoffs and Data replication in Cassandra
Your understanding is correct that beyond the default 3 hours, hints are no longer stored. This is to avoid overloading the other alive nodes for prolonged period of time.

If any node happens to be down for a longer period than the hint_window, its absolutely necessary to rebuild the node or even remove all the files from its data directory and let it bootstrap as a brand new node. You may have to use replace_address if you are bringing up the same node.