Let me reinforce this point mentioned by mushroommunk:

There was one game, No Mercy, that really did push things too far. Collective Shout was able to poster child that game and pressure payment providers into pressuring game platforms to delist a lot of NSFW games.

There were a few others and they were justifying and glorifying violence and terrorism. Not satire. Not as political commentary. Not artistically valuable. I recommend you don’t research this further. It’s real NSFL stuff.

They would be declared illegal, by any customer protection agency or national / government prosecutors doing their job. They were only online and available because those gov. agencies were not aware of them and/or too slow in taking them offline.

And because the steam’s process didn’t check them. Which Valve acknowledged in the sense that they admitted they didn’t and couldn’t and THEN took everything offline they couldn’t and didn’t check.

And THAT also hit unrelated stuff, because valve couldn’t check it and doesn’t want to take responsibility. Which, yeah, understandable.