Android's Direct Reply feature will be mandatory for OEMs moving forward, as Google has noted in changes to the Android Compatibility Definition Document this week. This means that smartphone and tablet manufacturers who make Android devices have to comply with Google's implementation of the Direct Reply functionality for notifications that show up in the notification shade. In addition to this, though, manufacturers will also have to support the rest of the behaviors that fall into this area of the way notifications are handled.
While Direct Reply may be the aspect of the notification behaviors that Google is referring to which people are most familiar with, Google's requirement also prohibits manufacturers from altering other elements, like removing the ability for the user to trigger the control panel from the notification shade. This also includes the user's ability to control the notifications as well, which includes being able to slide the notification to the right slightly to reveal a gear button which brings up a few different settings for the notifications for that specific app, like an option for silencing notifications, as well as an option for entering more settings.
Google doesn't make any specific mentions in regards to OEMs, but the new requirement is an obvious decision to make sure that all users can have the same experience on Android with notifications no matter their choice of device type, and it's an effort for Google to make Android just a little bit more streamlined in some of the more important areas. While manufacturers are still very much free to load on their own visions of a user interface for the platform, Google still requires certain elements of the OS to be untouched and the way users handle incoming notifications from the notifications shade is now now one of them. Of course, even if manufacturers are supporting these notification behaviors, the app developers themselves will still have to add support for the functionality to reply directly from the notification, and Google doesn't require app developers to do so, though it does suggest it as a recommended practice. That being said, even if every device manufacturer has to now support these notification elements, not all apps will allow them.