Right… but netplan uses systemd-networkd as its backend, and you can make override files to get the additional functionality. You don’t need to kill netplan completely.
To test this, I made an incus VM from images:ubuntu/24.04/cloud --vm; it has interface enp5s0. If I create /etc/systemd/network/10-netplan-enp5s0.network.d/proxy.conf containing:
[Network]
IPv6ProxyNDP=true
IPv6ProxyNDPAddress=2001:db8::1
Then do netplan generate && netplan apply, I see:
# ip -6 neighbor show proxy
2001:db8::1 dev enp5s0 proxy
# sysctl -a | grep proxy_ndp
net.ipv6.conf.all.proxy_ndp = 0
net.ipv6.conf.default.proxy_ndp = 0
net.ipv6.conf.enp5s0.proxy_ndp = 1
net.ipv6.conf.lo.proxy_ndp = 0
(and there’s no need to edit sysctl.conf either)