Windows Timeline and Sets are indeed two of the most anticipated Windows 10 features in a very long time. Terry Myerson previously detailed the two features would be coming in future Windows Insider builds, but today during the Windows Insider webcast things were clarified a bit more. Microsoft's Jason Howard confirmed you can expect Windows Timeline, but not the Sets feature, once RS4 hits the mainstream public.
Though it might be shocking for some, this was in fact previously hinted in the original blog post by Terry Myerson which announced the Sets feature. Windows Insiders can instead expect for the Sets feature to be A/B tested in upcoming RS4 builds, and see the feature get pushed out, then reappear in time for the following RS5 release.
Windows Timeline though, will stand, and Windows Insiders can expect to try out the feature in upcoming new builds. You can tune into the Windows Insider Podcast now for a deeper look at Windows Timeline.