I'm honestly curious. What line do you think the left crossed first? There is nothing in my mind that comes even close to the lines that have been crossed by this administration.
Just that line alone makes me think you're far less centrist than you claim to be.
This response is rich in vibes, low in fact, but my guess is OP is generally referring to things like the FBI's storming of Branch Davidian compound in Waco, the infiltration and surveillance of organizations, basically right and far right, in response to the OKC bombing, in the name of preventing domestic terrorism.
Go farther back and we have Hoover building kompromat on basically everyone, farther still you get McCarthy anti-Left. I don't think there's an era without any kind of corruption or abuse of power. It's romantic to think there was ever restraint, and like believing so seems to depend on starting the clock at an aribitrary historical point.