Didn’t realize & see upon updating that iOS 14.5 comes with “allow apps to request to track” disabled by default, which is much more of a shutdown than merely requiring Facebook to prompt users to opt in to targeted advertising. Most will never see such a prompt in the 1st place.
It's possible that I had already turned this off pre 14.5, in which case what I am seeing doesn't necessarily mean this comes disabled by default for other people who had not don that, plausibly postulates @bzigterman.