As per the Launchpad Source Package Page, the package does not exist in Yakkety or later.
root-system is not installable by default. You are welcome to download the Xenial packages and try to use those, however you should keep in mind that the package in question was dropped in Debian first, for the following reasons (Italics indicate something I added for clarity, and bold indicates important points which I want to emphasize; neither are part of the original bug report in Debian):
Dear FTP masters,
Please remove src:root-system and its binaries from the archive. ROOT is a huge beast to maintain, and it's currently not maintained:
- We have a quite ancient release,
- It's FTBFS (Failing to build from source) with newer libraries (as it's not keeping up with transition, and it's entangled in several of them),
- At the moment it has 3 RC bugs, oldest one being more than one year old about it FTBFSing with GCC-5.
Given these, I don't think it's wise to continue to try and use it, especially since there will be potential build errors with the newer libraries that come in Yakkety and beyond. It was dropped in Ubuntu because it was removed in Debian (you can see this by expanding the latest entry on the full publishing history on Launchpad, which points to the aforementioned Debian bug as the source reason)
Note that these fail-to-build problems here may also explain why there are no PPAs that contain the