[w7]

Yes, all implementation that I'm aware of use EUI-64, which requires a /64

https://datatracker.ietf.org/doc/html/rfc7421 is helpful in understanding this.

Basically it explains that SLAAC RFC itself does not define the /64 limitation, but other RFCs that are relevant to network operation do.

""" The addressing architecture [RFC4291] [RFC7136] sets the IID length at 64 bits for all unicast addresses and therefore for all media supporting SLAAC. An immediate effect of fixing the IID length at 64 bits is, of course, that it fixes the subnet prefix length also at 64 bits, regardless of the aggregate prefix assigned to the site concerned, which in accordance with [RFC6177] should be /56 or shorter. """