If Github wants to pass on solid talent just because of that person's race or sex, that's their (foolish) decision. It's a seller's market, and that talent will go elsewhere.
That's not really what I'm asking, though. Yes, let's say it's not actually hard yet. Is it reasonable to ignore signs that things might get to that point and wait for if and when it does?
I think it's reasonable to worry well before that.
(I don't know what if anything we can or should do aside from worry.)
I don't know what your definition of major is, but Github is not it in my book. Don't get me wrong, they make a semi-decent product, better than anyone else at this point, but they are by no means the only solution in their market.
The problem will self-correct at some point once the lawsuits are filed and the cost of such efforts is visible to executive leadership and investors. Even if they settle the suits early on, it'll still be a very expensive lesson. Workplaces with problems like Github is apparently developing rarely get better on their own without first experiencing the consequences.
Github is a major software company, and there are similar signs at other major companies, most of whom are starting down a similar path.