There are a lot of examples of this and the left has had some truly great advocates for free speech. In terms of time period, the Red Scare and Mccarthyism was a time when the left was being heavily censored by the right. The Civil Rights movements as well with MLK during the 1960s and then Frank Kameny in the 1970s trying to get rights for gays.

Other leftist advocates for free speech include Obama, Elenor Roosevelt, and Aryeh Neier are brilliant examples.