From the HTC support post (after a little translation!):
Following HTC and Microsoft R & D department investigation after repeated communication on HTC 8X / 8S upgrade to Windows Phone 8.1, the HTC 8X will be upgraded after push notification system testing is stable, while HTC 8S won't be because some hardware upgrades do not meet the criteria. The 8S will not be upgraded, hope you HTC 8S friends can understand.
From our earlier review of the 8S, back in the day:
The HTC 8S has just 4GB of internal storage on the spec sheet, and frustratingly only leaves roughly 1.1GB for user files. If you have any intentions of using the HTC 8S as a music player, to watch videos, or to do some picture taking yourself, then you'll need to get yourself a decent SD card as quickly as possible. As well as any data that applications need to store on it, the OS uses the internal storage for a number of temporary files and caches, which will start to fill up that single gig rather quickly.
...There is one new application, though, and it's one that upsets me. Called 'Make More Space', it's a utility that will look at the internal storage on the 8S, show you how much space the three main areas are taking up (music+videos, pictures, and documents), and allow you to jump to those hubs and delete content. It's a sad state of affairs when your newest app on a handset is designed to get around the biggest issue of the specification list.
Indeed. File the 8S under 'what were HTC thinking?'.... I maintain that it was effectively a dead device at launch, worse even than some of Nokia's misteps (think N97) and now officially dead in terms of updates too....
Yes, there's a microSD slot, but without sufficient internal working space to 'ground' the OS, sadly.