In case anybody was on the edge of their seat, I have an important update on the status of my League of Legends account! Turns out it was, in fact, COMPROMISED! It was accessed by a nefarious third party! I dunno what they did to get me banned, and doubt I'll ever find out unfortunately, but at least I've cleared my besmirched name, and (I think) regained access to the account.
The email/address that I signed up for the account was the same one I used for a throwaway Steam account that kept getting login attempts from around the globe for years until I realized what was happening and changed its password. So I assume that information is just on some global list of email/passwords that got mined during a hack years ago. Fortunately I don't think I have it linked to anything actually important, but I wouldn't rule out the possibility that I get more suspicious emails from f2p games I signed up for years ago and don't play anymore.
quick edit: I checked my match history on the account and it doesn't appear any matches were found. I imagine whoever got into it just tried to perpetrate some kind of fraudulent activity related to purchases and was likely foiled by Riot, hence the ban. My best guess anyways.