Explorer for ArcGIS continues on IOS and Android. While similar in name Explorer Desktop and Explorer for ArcGIS are separate software products.
For the latest on Explorer for ArcGIS check out this blog article Announcing Explorer for ArcGIS Beta | ArcGIS Blog
No concrete plans at this time. If it is developed, it will be developed for Windows 10 and later. At this time the product team is not seeing enough demand for Explorer for ArcGIS on Windows, when measuring against the demand for Android and IOS.
If I can bundle the data with my application for desktop use, it's always there. I don't have to worry about outages, slow internet, lack of internet, etc. Great! Develop a desktop app using an EDN account (which still costs money) and use the runtime SDKs! Well.......I'm doing this for philanthropic reasons. I can't afford to pay licensing fees, let along know how many licenses would be needed.
It's not that I'm developing some uber robust GIS application. To the contrary, it's a fairly basic application but I feel that the desktop Explorer + SDK might have been the best path for me to proceed with what I want to accomplish. I'm not the majority, I know, and ESRI has to maintain it's "positive growth". It's just......sad.