Here's [1] an overview of a bunch of programs. It includes data on how they've affected employee composition and discusses why certain things fail or succeed. The first success it cites is voluntary diversity training. The sort Google has. The sort that James Damore attended and then got angry about.
As said before: remove any selection bias, then hire those who can do the work, want to do the work, and have shown to do the work well before. Then measure performance and promote using the same objective processes. That's it.
Perhaps we can boil down the issue as the difference between making hiring as fair as possible, or making hiring ensure a certain outcome. The first option is good since it produces fair results, but the latter is actually what's happening in most places.
Diversity-based policies currently are only focused on the outcome, but the outcome is not what should be designed for. The outcome should just be what it will naturally be (whether it's "diverse" or not) and we should only control for selection and opportunity. This is what diversity-based policies do not help since you cannot work backwards from the outcome, you must start with making a fair process and just let people do what they want do beyond that.
That has nothing to do with diversity based on appearance nor will those policies help.