Version 2.1 of DD4T has seen many improvements and changes with respect to the DD4T 2.0.X branch. While most changes are compatible with 2.0.X, it is recommended to upgrade your web application to the 2.1.X version. The main reason is that the 2.0.X releases will be discontinued and active development on it has stopped. Additionally, new versions of the DD4T Template Building Blocks may push new data formats or properties to the front end and these will only be incorporated in the DD4T 2.1 branch. New implementations are further encouraged to use the latest version, even if you already have published DD4T 1.0 XML or DD4T 2.0.X / 2.1.X Json in your Broker database, in order make use of the latest support for libraries, Tridion and fixed issues.
As is tradition by now to release new full versions in the last days of the year, the end of 2017 brings again a brand new release of DD4T and it sure is by now a battle hardened, try to kill me but you won’t be able to release in full use at quite a few high volume and high load production environments.