From the article over at Nokia Power user:
We have expressed our doubts about Microsoft skipping on Windows Phone 8.1 GDR2 earlier too and now it seems more or less confirmed.
As per our sources, Microsoft is going to push Windows Mobile 10 or Windows 10 Mobile preview to “Insiders” pretty soon. It will be announced during 21st event. This in fact is in line with the original schedule of Windows Phone 9 (Windows Mobile 10 now) that we had reported long time back.GDR2 was supposed to be out in October but it was later scraped it seems and some of its tipped changes like “Mobile Data Toggle” and Cortana for more arrived with GDR1 version 8.10.14219.314.
Now, it seems the broader hardware support and other UI changes that GDR2 was supposed to bring will come with Windows Mobile 10 preview. So, GDR2 is mostly done away with unless there is some change in plans later. For your information GDR2 update has been ready for sometime now and skipping it means Microsoft has to do away need for “one more cycle of update” after Lumia Cyan and Lumia Denim.
The 'Insiders' bit refers to a public application that doesn't have a use just yet, but which presumably will tie into any programme announced on the 21st. Watch this space.
And, as NPU itself noted, with at least a preview/Insider version of Windows 10 for phones arriving in the next few months, any speculation about a new Microsoft Windows (Phone) flagship for the Autumn might effectively be moved forwards in time - maybe to MWC 2015? Personally, I'd put my money on 'just' the budget Lumia 435 and 1330, but I'd love to be proved wrong.
Even if an 'Insider' version of Windows 10 for phones does become available, it would be a very early alpha and I still wouldn't expect a stable release much before late summer or Autumn still.