How do I view cached PMTU under Linux (and all other OSs)
Maybe
ip route get to 212.58.244.69
Under Windows, use the netsh command to view the "destination cache" which holds this information. For example (assuming IPv4):
netsh interface ipv4 show destinationcache
MSS should be 40 bytes less than your MTU (it doesn't include IPv4 (20 bytes) and tcp (20) byte headers). So that is correct.
ICMP fragmentation needed message is sent by the router, not your server.