Thread was announced about two years ago, as a new IP-based wireless protocol based on 6LoWPAN and 802.15.4 standards and targeting IoT applications. Nest Labs, an Alphabet company, has now released OpenThread open source implementation of the networking protocol under a BSD license.
The source code (C++) includes supports for End Device, Router, Leader & Border Router roles, and can be found on Github. The implementation is said to be OS and platform agnostic with a radio abstraction layer, have a small footprint, and implement all Thread networking layers, namely IPv6, 6LoWPAN, IEEE 802.15.4 with MAC security, Mesh Link Establishment, and Mesh Routing. To quickly get started you can may to read the Examples README which explains how to build the code, start two nodes, and ping them.
Interestingly, while the code is there for everybody to use, only paid members ($2,500 to $100,00) of the Thread group can access the full specifications.
Thanks to Nanik for the tip.