std::net::lookup_addr was previously deprecated because:
reason = "ipaddr type is being deprecated"
IpAddr is not deprecated anymore and stabilized, but lookup_addr is now removed in nighly.
What's the reason for not stabilizing that function? The original cause for deprecation is obviously not true anymore.
Removed here, in a big deprecation purge. Pretty annoying as it seemed from the tracking issue there was still an open question, and at the least there was going to be some official effort to stand up an external crate. Nope.