Apps can't draw on top of the System UI in Android O thanks to a small change that Google is making to the TYPE_SYSTEM_OVERLAY code in the OS. This has now been changed to TYPE_APPLICATION_OVERLAY according to the Android Developers Blog. This change affects any applications that use a specific permission called SYSTEM_ALERT_WINDOW, which might not be a large selection of apps but there are some which will certainly find it more difficult to operate and may likely have to make some changes to the way their apps work.
Due to this change any apps which use the new "type application overlay" will end up rendering under critical system windows. So things like the status bar and the IME, which is the input method otherwise known as the keyboard, will all render on top of apps which now have to use the new overlay. Apps like Twilight which add a colored filter to the display so as to protect your eyes from strain due to blue light, will now render under the status bar and the keyboard. If you find yourself reading with Twitlight enabled for example and you end up having to use the keyboard for any reason, since Android O will cause it to render on top of the app the keyboard won't have the filter applied to it, which sort of defeats the purpose.
This change appears to have been made by Google intentionally, though the post on the Developers Blog doesn't explain the reasoning for the change. having said that since the change likely won't affect too many apps and hinder their ability to get their jobs done many users might not even notice any differences, and if they do they might not be bothered by the changes in the long run. This is one of the latest changes to surface that has been made with Android O and this is so far just the first preview build as there will be three more before the final software build that Google sends out. Other recent changes that were discovered as part of Android O include the support for automatic notification timeouts.