In our latest measurement study we use data from 350 volunteers combined with aggregate network logs from 60M ISP subscribers to show that around 90% of tracking flows originating within Europe also terminate within Europe.
This of course does not preclude that the collected data is not subsequently moved elsewhere but at least we know that the tracking end points are more frequently than not within reach of European Data Protection Authorities.
An optimistic result contrasting prior belief that tracking flows went straight out of Europe. For more details check:
C. Iordanou, G. Smaragdakis, I. Poese, N. Laoutaris, “Tracing Cross Border Web Tracking,” ACM IMC’18. [pdf]