They did, but that’s not where the reference comes from. It actually goes back WAY further to the 1800s, but that’s not super relevant (Scots in the Crimean War). The blue line has been referenced in the US for about a hundred years iirc, and had been a reference throughout PDs for themselves since around the 50s.

The movie I mentioned popularized the term outside of the PD, and the thin blue line flags started popping up as part of Blue Lives Matter in response to Black Lives Matter. Which is particularly gross to me, because it comes across as the police being in conflict with people of color, which just kind of proves the point of Black Lives Matter to me.

It took very little time for it to be a white nationalist symbol as well, in no small part due to many police also being members of the white nationalist movements out there…

So the flag is a response to Black Lives Matter, but the “thin blue line” has a much longer history that the flag is a reference to.