Supported parts include MSM8994, MSM8992, MSM8952, MSM8909, MSM8208, MSM8996, MSM8953, and MSM8998. This is basically the Snapdragon 810, 808, 617, 210, 208, 820, 625, and 830. Redmond also supports the APQ8092, APQ8094, and APQ8009 in Wi-Fi only devices that run Windows 10 Mobile Enterprise. This is the Snapdragon 808, 810, and 210.

All of the Snapdragon S4 SoCs are not allowed an official Windows 10 Mobile upgrade can be upgraded. However Redmond has bumped up the minimum system requirements to 1 GB RAM. Windows 10 Mobile does not support for anything that's made by MediaTek or any other ARM chip OEM. Still this is all a step ahead of what Microsoft used to do. Windows Phone 8 only supported the dual-core Snapdragon S4 chipsets. The Lumia 1020 was released with that SoC when Android phones were shipping with the quad-core Snapdragon 800.

However if all this is true, then why was the HP Elite x3, which has a Snapdragon 820 delayed until Summer? Microsoft says that the phone is supported now, so there is no real reason for HP to delay the launch.