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.