I don't intuitively understand how this change blocks targeted advertising outright. Sure, Facebook will have less information about me from other apps that I use, but certainly it has numerous other ways of targeting ads to me.

Facebook knows my demographic because I provided it to them when I signed up and the types of content I interact with in their apps, and they have both coarse and granular location (IP address and the location permission) from their first party apps. Most relevant forms of targeting seem to still work. Ads from the local florist can still be targeted.

It's credible to me that other, Facebook specific, magic such as "lookalike audience" and other behavior cohorting tools may be impacted by this change, but I don't view this narrow issue of IDFA consent to be an absolute judgment on ad targeting.