EXPERT RESPONSE
There's a simple answer to this: All subnets use a /64.
Because IPv6 provides a subnet ID as a part of the prefix, as opposed to IPv4 where you have to "steal" it -- so to speak -- from the host bits, there are no variable-length subnet mask (VLSM) considerations. Address design is very simple. It's also important to have the /64 as a maximum prefix length because you need 64 bits of interface ID for host addressing to work correctly (address autoconfiguration).
Some people have been using longer subnets for point-to-point links, but common practice increasingly is to use /64 even for those.
Answer provided by routing expert Jeff Doyle.
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