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Why?

It makes no sense you rarely see twice the same person in diamond downwards even queing back to back games yet in high elo where there are some undesirable egotistical schmucks I'd rather never have the "pleasure" to play with the feature is disabled when it's literally made for this.

What it the thought process behind this???

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about 2 years ago - /u/riotBoourns - Direct link

The problem is that at high MMR too many other abuse cases crop up that don't relate to avoiding someone who is being disruptive. You can use the avoid list to manipulate the matchmaker to provide more favorable matches for you. And high MMR players are too damn smart, y'all find all the edge cases and use them to gain an advantage. If we could have safely enabled this for higher ranked we would have. We would be open to increasing the rank threshold if things look really healthy for Diamond, but our simulations during development suggested this was too risky to ship with.