It's merged : https://github.com/ssbc/sips/blob/master/010.md

We need another SIP to define how forks work/ should work :P

very happy with how these specs are coming along

Threads and tangles are currently on track to become part of this repo: https://github.com/ssbc/sips

cc @Martín Dutra this is the thread you should reference. I'll weave this into the ssb-post spec I'm writing for a complete picture this week