Back in the days of yore, when Android 5.0 (Lollipop) was just gaining ground, root exploits were fairly simple, and usually packaged into one-click tools that tested a device's mettle until it broke, plain and simple. Given recent security troubles with just those sorts of exploits, however, they began to be patched in Android. Additionally, verification of a stock, unadulterated system partition at boot time has become the norm, with some manufacturers strictly enforcing it by not allowing a device to boot unless it passes muster. With Android 7.0 (Nougat), this strict enforcement is the requirement per Google, making systemless root in its current form the only way to achieve root on some such devices, short of modifying the kernel to not check the system partition. Other exploits do exist, but they often require modifications beforehand. Code in the upstream Nougat repositories that seems bound for the upcoming Pixel phones however, seem to indicate that even this will no longer work.
Current systemless root methods bypass modifying the system before boot by running all the requisite code in a space called the ramdisk, which is used at boot time by the kernel and normally not checked for parity. The Pixel-bound code, found by XDA member Dees_Troy, points to the ramdisk, kernel, and system partition finding themselves merged into a single file system. This means that the entire block will be parity-checked, leaving current exploits that rely on modification of the ramdisk out in the cold. While this can still be circumvented by a modification of the device's core bootloader, a number of flagship manufacturers these days are beginning to embrace a permanently locked and unmodifiable bootloader, and this trend could make its way to Pixel devices in the name of security.
While there is currently little indication that will happen, and a number of devices like the OnePlus 3 and some HTC devices still either ship with an unlockable bootloader or can be unlocked through a special manufacturer-provided tool, this change in the way Android works is disconcerting, to say the least. If you enjoy root privileges on your otherwise unmodifiable device such as a Samsung flagship, or would rather not risk messing around with your bootloader. Any device with a locked bootloader and Android Nougat and up is already essentially out in the cold, and the core system changes that look ready to be put to work in the Pixel devices don't paint any prettier a picture for root advocates.