If you are an employee of Riot then I guess I'd suggest you avoid playing in the competitive areas. It seems like a zero sum move: your employer is monitoring you to an inch of your life (which would probably be considered unacceptable in most other workplaces!) and to win you must play in a toxic environment where it is likely you might react badly and thus be targeted by Riot's HR department.

If you must play for your job, I'd suggest doing one of two things: a. Don't play to win, play half heartedly and don't engage, take lots of breaks and report each and every negative encounter to your manager so they can see that it's potentially impacting on your mental health, or b. speak to management about the toxic environment you are seeing and work with them towards studying what the chief causes are then start putting measures into place to prevent it.

Judging by Riot's job ads and glassdoor notes, playing LoL is required to get a job there. From what I know there is no gameplay in this game other than competition.

We do hire people who don't play League of Legends, though we are looking for people who are passionate about games LoL or others.

That's good to know. The job ads imply otherwise (I posted a quote elsewhere in these comments, and this is the new language, I remember it was saying effectively "must be an LoL player" few years ago). Not sure if you care, but I am not the only one who gets this impression and excludes Riot from the job search.

Thanks for the feedback, I'll let the recruiters know.

It's actually far more simple: follow Wil Wheaton's advice and don't be a dick. I'm Bronze (not al Rioters are good) and as you can imagine the toxicity is HIGH. That said it's not hard to not be toxic. Everyone has bad days, and you can't take it personally if someone on your team tilts. If it gets really bad just /mute them.