2.0.0
The main feature of 2.0.0 is Redis support.
The major version is increased because the binary representation of session locking metadata that msm stores in memcached was changed, which is only relevant if session locking for non-sticky sessions is activated. Therfore in these cases it's not a drop in replacement supporting a rolling deployment but requires a downtime!
Here's the complete list of changes from 1.9.7 to 2.0.0.