How would Steam know?

And yes, that's a problem that they're dealing with right now. Bellular News : Steam Faces Financial Obliteration: Others Are Already Dead https://youtu.be/AlDkL3DndtM

The law being talked about in the video is https://en.wikipedia.org/wiki/Online_Safety_Act_2023 / https://www.gov.uk/government/publications/online-safety-act...

That said, not all NSFW content is allowed in all jurisdictions. Australia and Japan (for example) have laws about particular content that differs from US laws.