You mean microSD and yes it's been removed. Supposedly it's never been necessary as the device will just download a channel that's not currently stored on the device replacing another channel. All it does is speed up the loading of channels, but it's always come with its own drawbacks (i.e., bugs).
I read the Wikipedia page on the Roku Models. The new Ultra with no microSSD has 4GB of channel storage whereas the previous Ultra has only 512MB for channels. So I would say no need for microSSD anymore.