After upgraded to Ubuntu 19.10 with systemd 242-6ubuntu1, the issue I encountered disappeared.
``` # 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 inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eno1: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 link/ether 94:c6:91:18:8c:3c brd ff:ff:ff:ff:ff:ff inet 192.168.50.249/24 brd 192.168.50.255 scope global eno1 valid_lft forever preferred_lft forever 3: wlp58s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000 link/ether 40:a3:cc:22:45:25 brd ff:ff:ff:ff:ff:ff 4: pppoe@eno1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000 link/ether 94:c6:91:18:8c:3c brd ff:ff:ff:ff:ff:ff ```
Now pppoe interface does not get IPv6 link-local address.
After upgraded to Ubuntu 19.10 with systemd 242-6ubuntu1, the issue I encountered disappeared.
``` UP,LOWER_ UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000 UP,LOWER_ UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000 NOARP,UP, LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
# ip a
1: lo: <LOOPBACK,
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
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eno1: <BROADCAST,
link/ether 94:c6:91:18:8c:3c brd ff:ff:ff:ff:ff:ff
inet 192.168.50.249/24 brd 192.168.50.255 scope global eno1
valid_lft forever preferred_lft forever
3: wlp58s0: <BROADCAST,
link/ether 40:a3:cc:22:45:25 brd ff:ff:ff:ff:ff:ff
4: pppoe@eno1: <BROADCAST,
link/ether 94:c6:91:18:8c:3c brd ff:ff:ff:ff:ff:ff
```
Now pppoe interface does not get IPv6 link-local address.