Microsoft publishes official (production) FCU device list
(All About Windows Phone)

Which smartphones are eligible for official (production-pushed) Windows 10 Mobile updates depends largely on their age. Which is why the Lumia 640 and 640 XL, hanging in there gamely for the Creators Update (CU), have now been dropped from the official list for the Fall Creators Update (FCU). Of course, there`s nothing to stop anyone with a Lumia 640/XL diving on the Insiders programme to get the latest Fast or Slow or RP ring builds of FCU and then leaving the programme so as to jump themselves onto the FCU branch for good and get the monthly updates. That`s the way readers here should go for any owned 640 devices, I think. Regular users will be fine with CU and won`t care, plus - if we`re honest - the two branches aren`t that different for Windows 10 Mobile for consumers.