While Android 5.x has been out for a while for the rooted HTC One M8 in various AOSP incarnations, and out more officially for the M8 Developer and Google Play editions with Sense as of a few days ago, it’s still a hit or miss on which rooted M8 devices one can install the upcoming Sense 6 and Android Lollipop update.
If you’re on Sprint, you’re out of luck, but you knew that already because you’re on Sprint. I kid. There’ll be a Sprint CDMA release soon, but it most likely has to come from the direction of Sprint this time, due to the special radio/EPST/dialer/koala interface.
In the meantime, the updates from HTC are in the hands of the carriers at what they call the Integration or Certification. For most carriers this means they’ll be injecting their bloat and branding into the ROM and claiming they’re not secretly modifying the kernel to add carrier-level spyware like CarrierIQ to the devices.
You can check your carrier’s official progress here. If you’re rooted though, there’s pretty much no need unless you’re on Sprint, or want to remain stock rooted with an OTA option on your carrier.