I'm using the setting ip6tables: true option with no problems so far.

The only conceptual difference I found to IPv4 is that no DOCKER-USER chain gets created, which makes it hard to insert custom ip6tables rules at a defined location.

dockerd --version: Docker version 20.10.17, build a89b842

iptables -nvL | grep DOCKER-USER
 417K  335M DOCKER-USER  all  --  *      *       0.0.0.0/0            0.0.0.0/0
Chain DOCKER-USER (1 references)

ip6tables -nvL | grep DOCKER-USER

Is this chain still planned / slipped through / not planned for IPv6? I could not really find any discussion about it browsing the old merged/closed tickets regarding ip6tables.