The unit system PR is still open for review: 

This is a big PR that touches in some way nearly every frontend and many other parts of the code, so I’d like to get about 5 approvals on it before we merge it in, especially from people that work on specific frontends. 

It has docs and tests, and there is a YTEP explaining the rationale behind it and what it does:

This should go in before 3.3 is out. 

Best,

John