So my theory about this decision is, that HERE was once build using many private APIs they could only use because Nokia had deep knowledge of WP8 as an OS and maybe was even capable of implementing some APIs on their own. Now that Microsoft took over that part of Nokia and HERE belongs to a different company, they have no more information on this kind of API.
Furthermore the old APIs they used are gone with the revamp called Windows 10. This is why the HERE Apps where initially gone for W10M. But Microsoft knowing about the importance of HERE to the Windows Mobile world and seeing, that a fix would require more than just a few weeks of work, introduced a temporary private API to Windows 10 Mobile th2 as a workaround with the goal to give HERE more time to develop a version of the App working with the public accessible APIs. Windows phone to android
Since this API was ment to be temporary from the beginning, it is now gone in the rs1 branch (and as a result the HERE App is crashing again on rs1). This is why I think they call the June 30 date – the rumored timeframe of the rs1 release