TELUS recently updated their update schedule to include the HTC One M8 getting Android 6.0 Marshmallow by December, and now it looks like the update itself is now officially certified.
Bluetooth SIG, which usually publishes certifications for major updates, now has a page for the HTC One M8. Once the update is certified it generally means it’s near in its final stages and is ready to be sent to the carrier for testing.
It’s also worth mentioning that the December timeframe given by TELUS is really just an estimation and the update could be delayed until January 2024.