It's very difficult. There are tweets that are gone, Facebook posts that are hidden or deleted, and in the case of G+, an entire platform gone (though admittedly I believe Mearls deleted that thread pretty quickly). The blog I got the post from is gone, and it took me going to the Wayback Machine to find it.
And these are the problems with asking for more evidence in these cases: as time goes on, it's easy for them to disappear unless people are actively gathering it or putting it in safe places. If Hill didn't save that screenshot, or if the Wayback Machine had not saved that blog post, I'd have a lot fewer specifics to work off of. But the thing is a lot of us (myself included) remember hearing about these things when they happened or after they did. However, I'm sure people would not take these memories of something happening as proof.
People say that nothing disappears on the internet, but that's not really true: a lot of things disappear on the internet all the time. It can be very easy to save things, but if you didn't realize you needed to save something, it can be gone forever before you even realize it.