Earlier this week, Microsoft's Gabe Aul announced that no Insider build should be expected for Windows 10 Mobile or PC before the company's October 6th event. However, he did inform users that build 10550 is being tested for PC and just a few days ago, new screenshots for build 10551 leaked, revealing enhancements for Microsoft's Edge browser. But things have been pretty quite on the Mobile front with the most recent build, 10536.1004, being released almost three weeks ago. Today, Gabe Aul has stated that Windows 10 Mobile build 10559 is currently being tested for the Fast ring internally.
It is not confirmed whether this will be the build which will be rolled out to Insiders, but given that build 10536.1003 was pulled at the eleventh hour - some people still managed to install it - little can be said for sure. Given the obvious lack of details, we can only predict that this build carries further performance enhancements and bug fixes - particularly the one which is unique to the Lumia 1020.
Just a couple of days ago, Microsoft celebrated the first anniversary of its Windows Insider Program, announcing that 27 builds for PC and Mobile had been rolled out in the past 12 months and that a total of 7 million people were testing these builds.
For now, Microsoft is temporarily holding off on builds due to its highly anticipated event on October 6th where it plans to unveil the Lumia 950 and 950 XL, an updated version of its Band wearable, as well as the Surface Pro 4 and its (renewed) vision for Windows 10 across those devices.