No, this has been a part of IPv4 for a very long time. The behavior is defined here. There are also differences in how the collapse white space, primarily in that IPv6 can collapse multiple zero sections
" a.b When a two-part address is supplied, the last part shall
be interpreted as a 24-bit quantity and placed in the
rightmost three bytes of the network address. This makes
the two-part address format convenient for specifying
Class A network addresses as "net.host"."
You're right. I'd never seen that usage before, but its in the man page.
