Yes it is. A packet being sent isn't reaching its destination because your ISP is choosing not to forward it? That we've come to expect that broken behavior is the reality that we live in, but a different route would be for the firewall/NAT device to forge an RST to both ends, since it will no longer be forwarding said packets on that TCP connection.
Given all the advances in technology, I don't think that's as bad an idea as it once was.
"choosing not to forward it" is an interesting phrase. NAT needs to have some expiry for each entry, because we don't have unlimited space for that table. Dropping the mapping entry has the same result as the other side becoming unavailable and is an understood state.
You can't just produce RST out of nowhere on the NAT expiry, because that connection may actually be active somewhere else. Consider a replicating pair of NATs - your connection gets moved from one to the other because (network reasons), but the previous one does not get a message about it because (network reasons). If it sent the RST packets it would actively kill live connections which it should not touch anymore.
Given all the advances in technology, I don't think that's as bad an idea as it once was.