Microsoft first announced Story Remix feature at Build 2017 and then pushed it to Windows Insiders for testing with Build 16193. But it seems not all insiders got the feature with Build 16193. Microsoft has clarified now that it happened because of A/B testing.
Story Remix is being rolled out to Windows Insiders in the Fast ring as part of A/B testing. A/B testing is a scientific method to prove the causality of a change on the observed impact.
While a large majority of Windows Insiders in the Fast ring should have Story Remix right now, if you do not see it (i.e. the Create pivot or search box), this means you are part of the control group.
But you can still get it if you don’t have Story Remix already. You need to send a mail to Story Remix team and you can get instructions on how to get it.
However, If you want to live on the bleeding edge, you can send the Story Remix Team an email for instructions on getting Story Remix early.