Hello! Thanks for reporting. For this particular issue, if your iOS 13 device can successfully connect using IPsec/XAuth or IPsec/L2TP mode, but the Internet does not work, it is because of the sha2-truncbug setting. Try the following fix:
Edit /etc/ipsec.conf and replace sha2-truncbug=yes with sha2-truncbug=no, save the file and run sudo service ipsec restart. Now try to re-connect.
Regarding the new TLS certificate requirements for IKEv2 in iOS 13, I'll look into that separately when I get a chance