No we've known and said it for a long time but not very loudly. People haven't used them sparingly and they tend to infect a codebase. Now we're just making a more concerted effort to communicate more broadly that they can be bad.
If you could do it again, would you leave in mixins or would you keep them out?
I think if we had to do it again (and get to keep the knowledge of the last few years), we'd probably skip mixins. We'd surely do some other things differently too though :)