Does ATT opt-in kill the ad monetization waterfall?

Long term, I think the ad mediation waterfall will die out, also as a result of everything ATT related, but also because this is the direction we have been moving towards for a while now. But these things take time.

While some of the largest ad networks are still not available for bidding platforms, my guess is that we will still rely on waterfall at least for the coming year. Good to ensure some competition with the bidding networks.

Expecting more and more of the impressions to be filled by bidding as time goes by, and hoping that the networks that are holding back on jumping on bidding on the mediation platforms soon.

If you mean the algorithmic waterfall that chooses what network to serve an impression based on historical results, then I think that's already well on the way out regardless of IDFA. A combination of eCPM floors and bidding already renders it pretty much obsolete in my opinion.

If by waterfall you also include the eCPM floors for various networks, my best guess is that the waterfall of eCPM floors will still be necessary for best results for at least the next 12 months, because some of the strongest performing ad networks are not going to be available on other platforms as a bidder in the near future.

We have the likes of MAX (by Applovin), AdMob, MoPub, and Fyber all offering mediation and strong ad networks. But AdMob bidding won't be available on other platforms any time soon, and likely the same with most of the others. Depending on what Unity does they may also be falling into the same bracket. So long as this standoff continues a hybrid solution is going to be necessary to get the best results.

I know at least some of the mediation platforms allow for IDFA and No-IDFA setups for each ad type, so having that set up will also be a standard requirement to get the best optimization.

I don't think so. There are still benefits to having a waterfall, perhaps even more so than before. With networks siloing more of their data, ecpm deals are still useful. I'd bet on a hybrid model.