Millennial here, been here since 2009. Posted here heavily from 2010 to 2017. Checked in now and then, but literally came back after being gone for two years yesterday.

My understanding of the timeline is this:
2004 - 2005: tiny site for a few weebs
2006 - 2007: expands, meme culture develops
2008 - 2014: rapid expansion, each board develops a different culture and demographic. Anonymous is broadly libertarian, and maybe even left-leaning, and becomes associated with hacking projects.
2015-1016: Many anons are into the Trump campaign usually because they think it's funny. The Neo-Nazi forum Stormfront appreciates the racist humor of /b/, and users start using /b/ heavily. 4chan re-establishes its politically incorrect /pol/ board as a containment board, in order to crack down on racism across the site. /pol/ quickly overtakes /b/ as the most popular board.
2017 - 2018: Racism on /pol/ escalates, mass shooters post manifestos and people post terrorist threats that they act on IRL, 4chan moderates better, and alternatives like 8ch and Gab become popular for extremists.
2019 - 2021: Racism on 4chan becomes so extreme that most normal people leave, and /pol/ becomes more conspiracy-brained, schizo-posting bizarre and idiotic theories
2022-2024: 4chan starts dying
2025: things normalize. /pol/ is still racist, but the other boards sort of go back to normal because the schizobrains basically killed themselves off