One of Microsoft's biggest flops over the past few years has been Windows RT, a specific version of Windows that was designed to run on ARM hardware, rather than the usual x86 hardware Windows had previously been built for. It was meant for low-power tablets that would primarily run Windows 8 applications, but due to a range of issues, it was never utilized outside a handful of devices.
The two main devices that ran Windows RT, and the only two that were even remotely successful, were Microsoft's Surface RT and Surface 2. Today, the company confirmed that these devices will not receive an update to Windows 10, effectively killing the platform once and for all.
Now it should be noted that Microsoft only confirmed that these two Windows RT devices will not be updated to Windows 10, and didn't comment on the (few) other products that run the OS. However if Microsoft themselves don't think it's worth creating a Windows 10 upgrade pathway for their own Windows RT products, it's unlikely they'd bother supporting any one else's.
Microsoft did reveal that the Surface RT and 2 will be upgraded to a version of Windows 10 that only provides some of the functionality. At this stage it's unclear exactly what functionality will be stripped out for these ARM-based products, but it paints a telling picture of where the platform is headed.
Going forward it seems clear that any large-screened Windows 10 products will be powered by x86-based processors, with ARM components reserved for phones and other smaller devices that don't require desktop app compatibility. Now that Intel is a strong competitor on the hardware side, both in performance and battery life with their Broadwell line, not having ARM-based Windows 10 tablets isn't a massive loss.