Emm... I thought tank update was about to appear during this PBE cycle

It is probably going to be the mid-year patch (like the mage update was).

mid season

They did not. Meddler said that they'd most likely release the information on who they are working on mid to late February. But even then we'll be waiting a bit before we see the actual products.

We probably won't be seeing them until May or later given how big of a task that group of champions is. They plan on going over the champions and their items that fit the class which takes time.