What does InetAddress.isSiteLocalAddress() actually mean?

The method is definitely not deprecated and it's definitely not just used in IPv6.

In IPv4 there are 3 network address ranges that are defined for site-local addresses: 10/8, 172.16/12 and 192.168/16.

Reading Inet4Address.isSiteLocalAddress() shows that addresses from exactly those 3 networks will return true on those methods.

IPv6 has a similar concept, here these addresses are called unique local addresses.

Effectively this tells you if the address you have is definitely not a public one (note that even if this method returns false, the address might still not be public).

Looking at the implementation...

For an Inet4Address, it checks to see if it's one of the RFC1918 "unrouteable" addresses: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.

For an Inet6Address, it checks the first two octets to see if it's a real "site local" address.