A few days ago, I made a ticket on riot support asking some questions about Vanguard on LoL. I will post Linux related questions in this post. The full questions and answers will be in the comments.
TLDR: Vanguard is not coming to MacOS (implied in answer, ~~awaiting confirmation~~ Confirmed, Vanguard is not needed to play LoL on MacOS), Riot doesn't know whether unofficial Linux support through proton will continue. Get your Darling ready!
Q: LoL currently works fine on Linux under proton, but with the proposed changes, LoL will stop working on Linux. Although Riot has stated that they probably will never natively support Linux, Riot has worked with the Linux community in the past to make LoL function under proton. Does Riot plan on continuing this line of communication with the Linux community to attempt to bring Vanguard to Linux as well (maybe not natively), or will Riot cease to support LoL on Linux at all? (even unofficially?)
A: As far as Linux goes, virtual machines have almost never been a supported practice as far as we know, and they're known to cause crashes and issues in general. The only support was the one you mentioned - proton, and we can't say for certain whether or not it will be a continued one.
Q: There is currently a version of LoL for MacOS, but Vanguard is currently unavailable for MacOS (Riot’s reason for why Valorant is not supported on MacOS), what does Riot plan to do about this (Will Vanguard be coming to MacOS)?
A: Players using Mac will still be accountable for cheating, getting account bans if detected (but not hardware bans), and they’ll also experience match terminations if someone else is detected.
I've opened an issue in the Darling Project repository, letting the developers know that there will likely be increased attention due to this situation.