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>Wasn't there a statistic from github recently, that 75% of the PRs created in 2017, were done from Macs?

No, there wasn't. Github posts lots of stats but nothing about OS shares.

There was. Apple shared it in the WWDC keynote when talking about changes to Xcode.

There's a full transcript of the last WWDC keynote on Apple's website and it has zero mention of Github.