My VPN was working with Ventura but is not working with macOS Sonoma 14.3
How to create case to Apple?
[Re-Titled by Moderator]
Verify that your Mac doesn’t have UDP port 500 (or how that is mapped by NAT) closed by a firewall setting. That would block IKEv2.
Else I’m not sure that Apple would be listening to incompatibilities with third party software or third party protocols on connected devices. They might argue that other developers have to program to work with Sonoma. You can try, though.
Contact Apple for support and service - Apple Support
Contact your VPN provider about settings that may be needed for Sonoma compatibility.
As 14.3 is out freshly (5 days), it may have some unforeseen changes that slipped through alpha and beta testing, but I doubt that this point-release would contain a big rewrite of networking routines.