Here's the thing though; if your self-expression goes against Riot's rules, you will be punished. And it's nobody's fault but your own that you get punished when you know that your self-expression goes against the rules.
Which goes to this one simple point:
The mute feature is a short-term defense against toxicity in the context of a match. It is not an excuse for toxic behavior. You cannot say "I should be allowed to be toxic because there's a mute feature", because, regardless of the mute feature's existence, the rules clearly state that you're not allowed to be toxic.
To pull directly from the Terms of Use:
5.1. Can I troll, flame, threaten or harass people while using the Riot Services? (No. If you do, you might get banned.)
While using the Riot Services, you must comply with all laws, rules and regulations in the jurisdiction in which you reside. You must also comply with certain additional rules that govern your use of the Riot Services (the “Code of Conduct”). The Code of Conduct is not meant to be exhaustive, and we reserve the right to modify it at any time, as well as take appropriate disciplinary measures including account termination and deletion to protect the integrity and spirit of the Riot Services, regardless of whether a specific behavior is listed in the policy as inappropriate. In addition to the Code of Conduct, please review the Summoner’s Code for additional guidance on exemplary gameplay behavior.
The following are examples of behavior that warrant disciplinary measures:
v. Transmitting or communicating any content which, in the sole and exclusive discretion of Riot Games, is deemed offensive, including language that is unlawful, harmful, threatening, abusive, harassing, defamatory, vulgar, obscene, sexually explicit, or racially, ethically, or otherwise objectionable;
And if you disagree with this, then League is probably not the game for you.