Google's newest Compatibility Definition Document (CDD) has made it mandatory for manufacturers to enable on-device encryption on new handsets that will be shipping with Android 6.0 Marshmallow.
Last year, with the launch of Android Lollipop, Google introduced on-device encryption for the Nexus 6 and Nexus 9, but urged manufacturers to enable it on their devices. However, as noted by the Android Police report, the company changed its stance on the matter and simply recommended through the CDD that OEMs do so for new devices.
For device implementations supporting full-disk encryption and with Advanced Encryption Standard (AES) crypto performance above 50MiB/sec, the full-disk encryption MUST be enabled by default at the time the user has completed the out-of-box setup experience.
Now, Google has finally made it mandatory for OEMs to enable on-device encryption once the user completes the out of the box setup of their device. As mentioned in the latest CDD, it may not be possible to enable encryption on devices that are already in use with a simple software update. Hence, older devices may be exempted from the new requirement.
As of now, the recently announced HTC One A9 and the new Nexus smartphones will be the only handsets shipping with Android Marshmallow 6.0. Therefore, it will take some time until real world issues or benefits of the feature come into picture.