Just noticed that the Wikipedia article regarding "#IPv6 address" says "::/0 -> Default *unicast*" here: https://en.wikipedia.org/wiki/IPv6_address#Default_address_selection
Has anyone any source/RFC why it'd be unicast only? Mathmatically it'd be both #unicast and #multicast, wouldn't it?

That was actually a question I asked on the #IETF PIM mailing list in the past: https://mailarchive.ietf.org/arch/msg/pim/ULFLgUK2tBTGkFu6lv9jjknaMsk/
Without getting a definitive answer.