The cable stations always could legally speaking as they don't broadcast over the air and therefore they don't fall under the FCC's jurisdiction. For a long time the cable networks didn't allow it under their own standards and practices most likely because they didn't want to prompt the government from feeling the need to regulate. That slowly broke down over the nineties and more or less ended in the 2010s as HBO and other pay TV channels became the model of success in television

Couldn't say for sure, but it really took off with South Park in 2001.

https://en.m.wikipedia.org/wiki/It_Hits_the_Fan

The first instance of "shit" on American network television (ie not HBO etc) that I can recall was on Chicago Hope. I think it was Adam Arkin who was able to say "shit happens" in one episode. There was a bit of publicity about it at the time.

Chicago Hope also managed to show a female breast, sort of. There was an episode where a woman had one of her breasts reconstructed, and they showed the result. I assume it wasn't an actual breast that was aired, but a lifelike replica. Either that, or they got away with showing a real boob by pretending it was a fake one in the story.

First time I heard it was on The Daily Show (sorry about the ads), but in the archives, they bleep it for some reason. I think they just allowed it because it was the name of a book. Brickleberry was the first show I saw use it regularly.

HBO always has, if we’re including non-broadcast stations.