NAT64 and DNS64 (see RFC 6146 and RFC 6147) are widely deployed mechanisms that allows network operators to only provide IPv6 capabilities to end devices while still allowing them to access the IPv4 Internet. For example, they are used by T-Mobile in the United States and many other cellular carriers worldwide. The way they work is by translating specific IPv6 addresses to IPv4 addresses. The network operator can choose to deploy them with a custom prefix of their choosing, or they can use the prefix 64:ff9b:1::/48 which has been reserved with IANA. The current Private Network Access specification states:

If address belongs to an address block for which the Globally Reachable bit is set to False in the relevant IANA registry, then return private.

This would prevent the Web from accessing the IPv4 Internet using this prefix, because that prefix is marked as not Globally Reachable.