Have you ever successfully disagreed with somebody without insulting them? It's how productive debate, which I was hoping for with my initial response, actually works. Try it.
OP spoke in broad strokes. You're referring to specifics that weren't even included in this discussion, and they're very much exceptions. And even in regards to those corner cases, my original point applies: When you identify that somebody is abusing the chat system, repeating literally the words that they use is the worst possible reaction. The intersection in the Venn diagram of people who act that way, with people who would receive and retain an announcement warning them against that sort of behavior, seems extremely small to me.
Fortunately, as we've seen on these boards, folks who DO fall into that trap can often have it undone via a support ticket.
I'm generally in favor of more transparency on Riot's behalf, which is why I do partially agree with OP's sentiment. However, I'm also constantly surprised by the degree to which people attempt to circumvent the system to remain toxic, so I'm guessing Riot has their reasons for NOT providing transparency on certain issues. One hypothetical reason against this announcement: Anybody who continues to violate the rules in this way would immediately blame Riot for not making the announcement more visible, or at a better time, or any number of other excuses. In a f2p game like League, Riot needs to be careful with what blame they accept for the actions of their players, or else appropriate bans and other punishments become more difficult to justify.