It does.
Here is a console output right after a reboot.
As you can see the sysctl.conf persisted however I still have a IPv6 address.
But, what I forgot to mention, is that the configuration is applied after running the 'sudo sysctl -p' command.
root@NanoPiNeo2:~# cat /etc/sysctl.conf
[...]
vm.swappiness=100
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
root@NanoPiNeo2:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether 02:01:5b:46:21:95 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.102/24 brd 192.168.1.255 scope global dynamic noprefixroute eth0
valid_lft 86152sec preferred_lft 86152sec
inet6 fd68:a057:2b71:1:78cc:3d8e:4a3a:347a/64 scope global temporary dynamic
valid_lft 7121sec preferred_lft 7121sec
inet6 fd68:a057:2b71:1:a203:3b39:51e7:9a7f/64 scope global dynamic mngtmpaddr noprefixroute
valid_lft 7121sec preferred_lft 7121sec
inet6 fe80::a711:5254:f436:6b24/64 scope link noprefixroute
valid_lft forever preferred_lft forever
root@NanoPiNeo2:~# sudo sysctl -p
vm.swappiness = 100
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
root@NanoPiNeo2:~# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
link/ether 02:01:5b:46:21:95 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.102/24 brd 192.168.1.255 scope global dynamic noprefixroute eth0
valid_lft 86135sec preferred_lft 86135sec
So maybe the actual problem is that the sysctl.conf isn't being loaded on startup.
And/or '/etc/rc.local' is ignored.