Blocking advertising in Nougat 7.0

Short answer

Ad blockers do not work in Nougat until the developers make an update. A beta version of AdAway does the trick but requires to check the new "Systemless mode" in Preferences screen.

Other ad blockers will probably implement similar solutions

Long answer

This is marked Community Wiki for others to contribute. The explanation is not fully clear to me too!! Everything is well explained in #770

Root applications do not normally have access to /system partition, even as root. This because the system partition is readonly by design. However, with mount command you can easily remount a file system in read-write mode and do whatever you like. This is what ad blockers and most root apps used in the past.

But Google changed the way the command mount work, to keep it in simple words. In Android 6, there actually were two coexistent implementations of the mount command, so if you didn't support toybox you could fall back to toolbox.

That is the simple reason why Nougat requires all root apps to update themselves if they ever need (and why ever in the world do you need to be root if not?) to modify the system partition