$ apt-cache show dhcpcd
[...]
It should NOT be installed on systems where interfaces are configured
by ifupdown via </etc/network/interfaces> using the DHCP method.
[...]
$ apt-get source ifupdown
[...]
$ grep -rB2 'No DHCPv6 client software found' ifupdown-0.8.44
ifupdown-0.8.44/inet6.defn- dhclient -6 -1 -v -S -pf /run/dhclient6.%iface%.pid -lf /var/lib/dhcp/dhclient6.%iface%.leases -I -df /var/lib/dhcp/dhclient.%iface%.leases %iface% \
ifupdown-0.8.44/inet6.defn- elsif (var_true("dhcp", ifd) && execable("dhclient"))
ifupdown-0.8.44/inet6.defn: echo 'No DHCPv6 client software found!' >&2; false \
[...]
It's looking for dhclient, not dhcpcd.
In inet.defn:
method dhcp
description
This method may be used to obtain an address via DHCP with any of
the tools: dhclient, udhcpc, dhcpcd.
But for inet6, that's dhclient only.
It's found in:
$ apt-file search -F /usr/sbin/dhclient
isc-dhcp-client: /usr/sbin/dhclient
isc-dhcp-client-ddns: /usr/sbin/dhclient