League of Legends

It's reasonable to not want to run Vanguard. Just know that all the (valid) reasons for that also apply to many other games, including just about every first person shooter. And most of the reasons apply to literally any software.

Applications don't need root access to see (and upload) your "My Documents" directory.

If you're concerned about this, the solution is to use different operating systems for personal stuff and gaming (or other less trusted software). Dual booting Linux and Windows is a perfectly reasonable thing to do for this reason. Your less trusted OS should not have access to the sensitive data at all. Don't allow it to read your other drive. I'm surprised there's not a hardware switch to swap hard drives. https://www.phind.com/search?cache=wyp7j8oa2uk1aa1vr73b9v9r

Personally, I trust Riot and Blizzard with their access a hell of a lot more than random Steam games, and I still run Lethal Company, which is certainly more questionable.

If Riot or Blizzard are caught doing anything nefarious, they have a LOT more to lose than small indie publishers on Steam.