I think the most impactful thing this decade was the rise of the livestream. Livestreams are the gift that keeps on giving for those who watch trashfires and there were a number of infamous livestreams this decade. To add to that, livestream chatboxes featured crowds either cheering or mocking the streamer. Multiple times livestreaming has drawn people to this site, from the ParkourDude91 saga in the mid 2010s to the New Zealand shooting stream recording this year. This was really boosted by the fact that cell phones have amazing cameras these days and cameras for streaming are cheap.
Out of these probably the best single livestream was the multi week He Will Not Divide Us stream, which essentially turned into an IRL shitposting box. For a few weeks the stream featured characters of it's own, exceptional individuals on both sides making jokes of themselves, people fighting to get in front of the camera, and at least one e-celeb making an appearance there. After both locations ended up becoming problems for the property owners, the stream became nothing more than a flag...which ended up being stolen based on channers triangulating the location of the flag. There was also the controversial IRL streaming craze, which had crazy moments including the Arab Andy arrest, pretty much every Asian Andy stream, and Ice Poseidon's swift rise and fall. Worth noting as well were the Terry Davis streams, which were essentially IRL streams through the eyes of a schizophrenic programmer browsing the web. Before Terry died, some were debating on if he lived the most well documented life of anyone out there solely because he was streaming everything he did (which turned out to be the beginning of his downfall once edgy kids sent him porn).
As for the most defining meme I'd have to say the musical mashup trend, soundclown shitposts, and similar. Quite a few others have already named all the obvious defining memes anyhow, but mashup culture was quite huge worldwide during the 2010s. There was even a huge channel during this time that focused on mashups: SiIvaGunner. Plenty of bait and switch style shitposts were popular thanks to those streamers I mentioned earlier, as people would pay money to them to blast music. There was also the Vaporwave craze that continued to morph over time even as the original music genre that started it died out.
