This was the reason I moved from Windows 10 back to 8.1 on my 1520. But I noticed it is not in the store for 1520 (8.1) or 1020 (8.1) either. Which means if I uninstall it, I won't be able to re-install through the MS Store. I think before the official release of Windows 10 mobile for older phones one still can get it back by using the restore desktop app. But after the official release there probably won't be the way back to 8.1.
So I am going to stick with 8.1 for now just because those 2 apps: HERE Maps and HERE Drive. And then see what happens. If they never appear back for Windows 10, I am willing to try Android or something. It is funny that HERE Maps IS available for Android and IPhone. I use those apps since Symbian times, and MS Maps is a joke compare to HERE Maps.