Sure, but as the ancestor comment mentions, SLAAC has no relevance for the use-cases people in this thread want more then 64-bits for (e.g. Docker sub-networks)

Sure, but as the ancestor comment mentions, SLAAC has no relevance for the use-cases people in this thread want more then 64-bits for (e.g. Docker sub-networks). Since your Docker containers don't have "MAC addresses", there's no reason that you need to use 64 bits specifically to configure them. Your container runtime is perfectly positioned to assign IP addresses and subnets however it chooses to. Assigning a /72 to each container is perfectly fine.