>Sometime around the night of the DNC primary in 2015 there was a dramatic shift in the content on Reddit. If you were a frequent visitor of the homepage and /all at the time, you will know exactly what I am referring to.

Indeed.

It's especially visible when the effect temporarily abates. After Trump's 2016 election win, for a day or two, it was possible to post something non-leftist to /r/politics without having it be mercilessly downvoted; as if the shills were awaiting orders.