Is there any safe amount of alcohol?

Well, it’s like epsilon-delta proofs in maths, isn’t it? For any acceptable amount of harm (epsilon), you can find some upper limit on consumption (delta) that causes harm below that threshold based on our current experimental data. Presumably, most people won’t care if epsilon is small enough, for example smaller than the risk of getting cancer from a walk in the park without sunscreen on.

Is there any safe amount of walking down the stairs?

Stair walking isn't a group 1 carcinogen. It's also required in many living spaces.

If I understand correctly, «group 1» says nothing about the probability of getting cancer from exposure, except that we have sufficient data to be sure it must be nonzero?

(FWIW, I don’t drink myself, but I don’t consider cancer risk a motivating reason here.)

Yes, stairs typically kill you through blunt force trauma, not cancer. Your point?

My point is that most everything comes with risks and we trade them off with our desires or needs. The recent push to evangelise that there’s “no safe amount of alcohol” is rather silly in the light of that. There’s no “safe” amount of driving or cycling either but we do them anyway because they satisfy our desires and we judge those to be worth the risk. Alcohol (as long as one is not addicted) is the same.

My point is stair climbing isn't a relevant comparison. It's often required, and usually an activity that improves ones health. Alcohol--even in moderation--appears to have only negative health effects and is entirely optional.