We have also branched 2.1 and tagged Release-2.1.7-beta. This has many big changes (and potential compatibility breaks) compared to 2.0. I encourage you all to try it and report any problems. Please see the CHANGES.md in that branch for the full list of enhancements and fixes.
My plan is to let the beta span the course of November, with a goal of declaring a true 2.1 release on December 1, at that point 2.1 then becoming the fully supported stable release branch and 2.0 fading to be an obsolete branch that will only get sporadic updates to address the most critical bugs.
During the beta period, there may be some last minute touch-ups that change the API/ABI, but once we hit release candidates at the end of the month, the API and ABI will be frozen as befitting a supported release.
For the moment, master will continue to track the 2.1 betas, but at the moment that we have a PR that we agree should not be incorporated into 2.1 (because it's risky or breaks compatibility), master will diverge and truly be the pre-2.2 work area.