Here is a statement by the VVN-BdA on this topic: https://vvn--bda-de.translate.goog/wider-die-geschichtsvergessene-umdeutung-des-roten-winkels/?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp

Small text why Antifascist groups in europe use this symbol: "Antifascist groups use the red triangle as a symbol because it historically represents the identification badges used in Nazi concentration camps during World War II. Specifically, the red triangle was used to mark political prisoners, including communists, socialists, trade unionists, Freemasons, and other individuals who opposed the Nazi regime.

The red triangle has since been adopted by antifascist groups as a symbol of resistance against fascism and authoritarianism. By using this symbol, contemporary antifascist movements connect their struggle with the historical fight against Nazi oppression, honoring those who suffered and died in the concentration camps for their political beliefs and opposition to fascist ideologies."