The International Association of Privacy Professionals (IAPP) explains the nexus of Schrems II, Privacy Shield and Brexit.

“While the adequacy assessment for the U.K. is currently underway, a U.K. adequacy finding is by no means a given. Given that the EU-U.S. Privacy Shield appears to have been invalidated primarily because of concerns about U.S. law and practice on government surveillance, similar arguments could be made in relation to the U.K. adequacy assessment. This is particularly so in view of the broad powers of the U.K. authorities to intercept communications and require access to data under the Investigatory Powers Act 2016.”

Details from the IAPP.