I had to look this up. I think you’re referring to metamoderation where users rate the ratings of comments:

https://slashdot.org/faq/mod-metamod.shtml

Sounds like a great idea, I wonder why it hasn’t been adopted by other websites. Do you know if it had downsides for Slashdot?

The problem is that if you get hit with hivemind moderation in M1, then you're likely to see it rated as fair in M2. This is encouraged by the "Troll" and "Flamebait" options which can be slapped on any dissenting opinion.

> "Do you know if it had downsides for Slashdot?"

In a word, ossification. It preserves the culture of a forum, to a fault.