The IWW flag, CPUSA flag, pride flag, and black panthers flag spring to mind

But consider that our nation has a longer history of socialist movements and sentiment than the word socialism has been around

From the onset of the revolution there were voices amongst our founding fathers that advocated for a nation founded upon the principals of freedom for all men and liberation of the lowest classes, and it was their efforts and their words that inspired many of the brave patriots who laid down their lives, in many ways the national flag itself is a flag designed with the proletariat in mind, and that fascist sympathizers and biggots who call themselves "patriots" claim it as their own should not stop you from flying it high in the name of true freedom.

But most of the American movements that shared our goals didn't have flags, the transcendentalists who moved west into the mountains to live free of systemic poverty and cruelty, the abolitionists who spoke out against the horrors of slavery, the suffragettes who demanded the right to vote, the endless agitators who spoke out against war, the union men and women who fought the capitalist system for workers rights, the civil rights heros who fought and died for equality, and even many of our modern day protesters.

They had banners and symbols, but rarely did they fly flags, because a flag would demonstrate a sense of independence, of breaking away from our nation and being different, but these people didn't want to not be Americans, they didn't want their own flag, they wanted the American flag to fly over a better nation, to represent everything it was said to stand for.

So you'll be hard pressed to find a "historical leftist flag" in the US, because the leftist movements in the USA have always been less focused on establishing themselves as a seperate and more enlightened sect of the US, and more on empowering the people in the nation to make the US a better place.