Does Steam require an AI disclosure?

Yes, and Steam shows a version of it to buyers:

• Developers fill in an AI section of Steam’s Content Survey describing "how you are using AI in the development and execution of your game."
• Valve says it will "include much of your disclosure on the Steam store page for your game", so this is customer-facing, not just paperwork.
• Content made with AI while the game runs ("Live-Generated") also requires describing the guardrails that stop it generating illegal content.
• Valve reviews AI output the same way it reviews everything else, by a person before release.