Yes, unfortunately it is not hard to find routers without globally routable IP on a loopback or interfaces (whatever is used as the source for ICMP unreach in a

Yes, unfortunately it is not hard to find routers without globally routable IP on a loopback or interfaces (whatever is used as the source for ICMP unreach in a given configuration) but if they have any IP at all, even a private IP, PMTUD still may work as long as ICMP messages with private source IP are not blocked by a firewall. But when a packet is dropped inside a carrier network (and not on its edges) because of a MTA mismatch it looks like a gross misconfiguration to me.