The instructions at https://openwrt.org/docs/guide-user/services/vpn/ipsec/openswan/openswanxl2tpvpn shows only IPv4 support, does anyone have any guide to install complete L2TP server with IPv6 support.
I guess its not possible.
Or perhaps it is just that the people with IPv6 experience aren't using L2TP as, by itself, it's unencrypted and there are multiple, robust, encrypted, authenticated VPN alternatives available. Asking for server makes it even more unlikely, as generally the L2TP questions comes up when trying to be a client of a legacy system.
If you were looking for OpenSwan, you might check out https://www.openswan.org/
Android supports L2TP/IPsec PSK and L2TP/IPsec RSA (and some Xauth and Hybrid types as well) without using a VPN app, though I don't think it supports IPv6. (It wasn't supported last time I checked.) But IPv6 support would be useful. The StrongSwan App doesn't support IPv6 since it has to use UDP-ESP instead of ESP, and VPN server software usually doesn't support UDP-ESP over IPv6, It's for example unsupported in Linux.