We've heard from a few different places over the last few months that an EF 24-70mm f/2.8L IS is currently under development. At the time of the EF 24-70 f/2.8L II launch, there was a lot of talk that an IS version of the lens existed. We had then heard Canon planned to have 4 new versions of the 24-70 focal range. We got the EF 24-70 f/4L IS, which surprised a lot of people and then the EF 24-105 f/3.5-5.6 IS STM arrived, which we consider the basic focal range with a little bit of a broader appeal. We do have the EF 24-105 f/4L IS, but it's not part of the new lens designs at Canon.
With Nikon rumored to be finishing development on a Nikkor 24-70mm f/2.8 PF VR, it seems unlikely Canon would leave such a lens out of their lineup, a lineup they're very proud of.
We're also told that an IS version of the EF 24-70 f/2.8L II would not replace the non-stabilized version, and would probably cost a lot.
There is no timetable and some grains of salt are probably required with this one.