Whats the status of it now? I wonder what happened to that program and why is it still on the Store if the Insider Program replaces it?
The Insider Release Previews replaces it, why the Preview for Developers is still on the Store is anyone's guess.
So people with abandoned devices like the 810 can get Windows Phone 8.1 Update 1.
That still doesn't explain why you can install it on Windows 10 Mobile though.