They announced it officialy since the release of the IMVU Beta Client for MacOS, around 2 years ago. Once the beta ends they will shutdown the old client for good, forcing us to use their new shit that they are so proud of. I would gladly give you the link to that news if I had still had my imvu account. There are multiple reasons behind this decision and the top ones are probably to counter the rising of 3rd-party unofficial clients like T4 and the deprecation of Flash since the old client still use it. And as we know how imvu works since a while, when something goes wrong they simply delete it instead of fixing it. I could give many examples of this situation.
Anyway I don't give a shit anymore about them and I made a new life since on Second Life which is way better. Even some of my friends followed me there. Because of their stuborness and stupidity, imvu will loose all of their old customers. Only teenagers will be left on imvu since I heard they are targetting that age range anyway and they gonna make a new game only for teenagers.