Thanks for this useful and clearly written document.  It’s ready to go, and I only have two tiny comments that are both in the “use your own best judgment” category:

— Section 1 —

   A route towards an IPv4 prefix that uses an IPv6 next hop is called a
   "v4-via-v6" route.  V4-via-v6 routing is not restricted to routers,
   and could usefully be applied to hosts, but doing so would require
   solving the issue of host configuration, for example by extending
   either DHCPv4 or DHCPv6 to publish an IPv4 default route with an IPv6
   next hop, which is out of scope for this document.

There is no inherent reason that this is “out of scope for this document”, right?  It’s just a choice of which use case this document covers and which it doesn’t?  It’s a tiny thing, but I would phrase that differently to make ithat situation clear, perhaps thus:

OLD
   next hop, which is out of scope for this document.
NEW
   next hop.  As noted in the previous paragraph, this document has a
   specific focus, and the host case is not within the document’s scope.
END

But, as I say: a tiny thing, so simply ignore that if you prefer.

— Section 4 —

   If a router does not have any IPv4 addresses assigned, the
   router MUST use the dummy address 192.0.0.8 as the source address of
   outgoing ICMP packets ([RFC7600], Section 4.8, Requirement R-22).

This isn’t a requirement specified in this document, but one taken from 7600, so I would avoid using the BCP 14 key word here.  I suggest changing “MUST” to “is required to”.