According to recently rumors on CR, along with EF 85mm f/1.4L IS USM lens coming in this Summer. Canon is still working on the rumored Canon EF 24-70mm f/2.8L IS USM lens. The release date of this lens is currently unknown, could be announced in late 2017 to 2018. Previous development of this lens is not announced due to weight and size.
When EF 24-70mm f/2.8L II USM lens in development and released in 2012, there were prototypes with IS being tested alongside. The reason is that the weight and size is large that makes some sense once you consider the target market, which is generally event photographers that have to carry and use the lens for 12 hours or more on some occasions.
So Canon is still actively working on this IS version lens, to find ways to reduce weight and size.