Do Not Disturb mode is getting change alerts in Android P Developer Preview 3. More specifically, when any changes are made to the Do Not Disturb mode it will send a notification to the system and alert the user, so the user will see that a change has been made just as they would see notifications come in from any other application, like messages or social media for instance. What's more, is that the notification will allow users to tap on it if they're particularly interested in finding out what's been blocked by Do Not Disturb.
Another change with the third Developer Preview is that users only have two options for incoming notifications now in regards to the DND mode. While it's possible to silence incoming notifications on this build of Android P, when you silence them they're still shown visibly, so the notification will still pop up in the notification tray like before, just without sound. The other option is that you can turn notifications off entirely, so they won't pop up at all and won't make sound. That said both those options are likely to sit well with most users, save for those that want as many options as possible.
For now these seem to be the only major changes that Google has made to the functionality in this latest build of Android P, but as Google has been known to change things more often than one might think, there is also a possibility that DND mode could change a little more before the final release of the Android P software gets pushed out to users later this year. For the most part, what Google has done with DND mode is make it easier for users to work with by taking out what it has considered unnecessary options for the average user. This should make it more accessible to a larger portion of Android users.