It's giving it the latest OS of Win 10 that can run on an over 2 year old phone.
But that's just it. They aren't. The "latest OS of Win 10" that could run on the M8 would be actual Win 10, not the preview. From what I can tell, the M8 was among, if not the, fastest Windows Phone 8.1 devices released, and runs the preview beautifully. Presumably because of the low install base, MS/HTC decided not to pay carriers to test and release Windows 10 for it.
That means, I believe, that the M8 is effectively abandoned. I have to imagine iOS upgrades at least include two generations before, much less one generation (two years old or not, the M8 is the immediately previous generation to Windows 10, and lesser phones are receiving the upgrade).
All that said, I installed it last night, and it's running like a champ, even without a hard reset. If I knew it was going to be so seamless, I would have done it months ago! My about screen reads:
Version 1511
OS Build: 10.0.10586.107
Am I on the best/latest build for HTC One M8? Has anyone on the M8 seen their version magically start getting post-release builds?