Update #2 (01/15):The HTC 10 Oreo update appears to have been pulled. The RUU file was previously made available for download from HTC.com, but its page can no longer be accessed. The over-the-air update was also thought to be rolling out to unlocked devices in the US, though we haven’t come across any reports of this in the wild.
The reason why the download was withdrawn, or indeed why it was made available in the first place, remains a mystery. >Android Police suggests it was only HTC 10 owners with Sunshine S-Off—a paid-for boot unlocking tool for HTC devices—that were able to flash the RUU file, while those without it had tried and failed to do the same.