Currently many routers (I’ve tested and written a
tutorial on Ubiquiti, and OpenWRT) allow you to set the subnet you want. In addition to
that, some ISPs have pages that have details on how to do this in FreeBSD, OpenBSD,
Gentoo, Windows, Cisco, etc. Of course, the modem / router / switch / access point
provided by the ISPs may not allow you this, but it’s certainly possible.
In the MikroTik routers quite popular in amateur networking, and also in the AVM Fritz!box
routers that my ISP provides, it is not possible to do this when DHCPv6-PD is used.
(it is possible when static IPv6 addresses are used as is done by some ISPs)
There is an open feature request at MikroTik to implement something like you depict in
your tutorial, but currently it is not there.
Rob