We were interested to test Roku Ultra (check here for pricing on Amazon) and its Miracast wireless display capabilities which had been enabled on its predecessor, the Roku 4.
Sadly, once we went to settings -> System there were no more options for Screen Mirroring which is apparently no longer supported on the Roku Ultra but had been supported on the Roku 4 (as can be seen in this review).
Perhaps Roku felt wireless screensharing was a popular enough feature for them but Roku fans that need that feature will need to stick with the Roku 4 (click here for Amazon pricing) for now.