An easy pick for me but it captures the last decade so well. Lincoln looking at the noose/halo is what sends this one over from being just a funny picture into being genuine art. It should be in a gallery. It should be in the entryway to the White House.
2015 was definitely when it kicked into high gear, when you could say that the trump era formally began with him announcing his intention to run with the speech about how "they're not sending their best".
i do think it - however you want to define "it," but i think we both know "it" - started gaining momentum a couple years before. i'm 27, so i was a teenager in 2013 and didn't really have my finger on the pulse, but even in the gaming sections of the internet that i pretty much exclusively hung out in around then i remember feeling a shift beginning. the reaction to games like Gone Home and depression quest felt different than it would have if they had come out a few years earlier. gamergate's importance can be overstated by some but i think it's a reflection rather than a cause of a wider rightward shift that was just beginning to gain steam then (2014).
in any case it seems like people think about the pre-trump 2010s the same way they think about 2019 vs 2020 or the late 90s to post-9/11. a time marked more by ignorance of what was to come than any properties the time had itself.