UHCP (micro host configuration protocol) is a RIOT-proprietary protocol
that was developed back in 2016 
that begs the question of why it's still default.
There's been some discussion on IRC on 2020-10-20 that concluded that
the remaining advantage is that the UHCP server can be host-built
I half expected dnsmasq to be used as a DHCPv6 server, but that seems
not to be the case; instead, dist/tools/dhcpv6-pd_ia has been added
Could, with some updates to the border router example setup scripts,
that or dnsmasq fill the gap?
PS. The DHCPv6 client is working really smooth; except for some numbers
that needed increasing and one hack to prefer routable over ULAs, the
DHCPv6 version of the border router example works quite straightforward
with OpenWRT routers -- plug it in, install the missing CDC-ECM kernel
module from the GUI, assign usb0 to your LAN and off you go with real v6
addresses being assigned to your devices!