Seems like just a few weeks ago no one took offense to the word "boomer" and now it is even being considered a racial slur. It actually IS a racial slur in Australia, where it is a pejorative against the Aboriginals as they would use boomerangs for hunting, although Wikipedia does not recognize that Aboriginal ethnic slur, but does recognize it as pejorative retort against outdated or condescending attitudes of older people, Even the US government affectionately referred to the age group in 2002 as Baby Boomers and noticed their would be issues with them
Know Your Meme explains it pretty well and also offers some interesting insight into the usage of the term preceding its viral climb to an established phrase and meme. Here are some of the earliest usage of the phrase in memes and comments before it went mainstream
[PUTS ON HIPSTER GLASSES]
On October 26th 2017, user Writerhaha used the phrase as part of a retort to someone who made some sort of out-of-touch comment.
Good ol' Twitter shows up late to the party and the phrase is tweeted out in April of 2018.
Of all places, the less-than-serious subreddit of r/OkBuddyRetard somehow nails the exact sentiment by taking the "Ok Retard" meme with Doge and substituting the word Boomer. A very veiled insult in the way it was slid in there. And as they say, the rest is history.
Just barely a week after the Doge OK Boomer meme came out, the meme had spread so far and wide that it was even uttered in an official New Zealand parliamentary session by Chloe Swarbrick.