Obviously (and ironically) the Cohen Brothers, the writers and directors of The Matrix created the concept. But they did not apply it to contemporary politics, that happened a bit later
It is easy to demonstrate that it was already in wide use long before the first Q post, which according to this compendium was onOct. 28, 2017
Here are a few links, with dates:
Vice Magazine, Nov 22, 2016: How to Tell if Your Alt-Right Relative Is Trying to Redpill You at Thanksgiving
New York Magazine, The Online Radicalization We’re Not Talking About
Illustration from the above article
Daily Dot, Sept. 1, 2016 : Donald Trump and the rise of the alt-right
Alt-righters will sometimes refer to themselves as “red pillers,” a reference from The Matrix that other online movements, like Gamergate, have also co-opted. To swallow the “red pill” means to accept the painful reality. In the case of Gamergate, the “red pill” was that men had to reclaim their dominance over women or risk their downfall. In the case of the alt-right, the “red pill” is that all races were meant to exist separately.
I have not done an extensive search to find the earliest use, my recollection is that it was first used by the "men's movement" to differentiate between the men who have thrown off the feminist brainwashing about the sexes, and those who have not.