The architecture of communication(evolutionistx.wordpress.com)
evolutionistx.wordpress.com
The architecture of communication
https://evolutionistx.wordpress.com/2015/11/12/the-architecture-of-communication/
https://evolutionistx.wordpress.com/2015/11/12/the-architecture-of-communication/
But I'm not sure if anonymous boards are the solution. Without reputation, quality can quickly go down, and there is no incentive not to be needlessly nasty.
I think the best solution is for executives to actively solicit contrarian opinions via 1:1 meetings. An engineer might not want to speak out about why some project is doomed in a meeting of stakeholders. But if the exec comes to them privately and says, "give me your honest assessment, if this plan has holes in it, I want to know now," then they are likely to get an honest answer.