My point isn’t Facebook in particular, but more generally around data residency. Inherently data must be saved and stored to all who access, no?

Wouldn’t the true solution simply be for those who don’t want other regions to save to simply not have access, e.g firewall?

My understanding was that it's about where the data resides. That assumption may or may not be true. But if it is it's feasible for EU data to be stored in the EU and US users request that data from the EU server. Again, my understanding was that this would be a valid way to handle that data but I might be wrong.

Your understanding is correct, but my point is that inherently if US people can access EU profiles inherently some EU data will be stored in the US.

It would then been trivial for someone to crawl EU pages and rebuild the corpus of data.

The only real solution is to firewall off regions and prevent those users from accessing anything other than their own.

This is already what some countries like China do, for instance

I don't think this is what the EU is requiring even though it is possible to scrape EU data. Facebook is pretty good at preventing scraping though.