Among other things, iPhones are revered for their privacy-first approach, with Apple famously refusing to break encryption even after multiple requests from the FBI in 2015. Alas, ten years later, Cupertino has capitulated, this time, due to incessant pressure from the UK Government. As a result, new iPhone users will not be able to use Advanced Data Protection any more.
9to5Mac spotted the notice on a UK-based iPhone, which said, "Apple can no longer offer Advanced Data Protection (ADP) in the United Kingdom to new users." Even existing users will have to disable the feature if they want to use iCloud features. But, it is opt-in, so you probably won't be affected unless you explicitly enabled it. This is also why you have to do it on your own because Apple lacks the ability to disable E2E encryption remotely.
But, this won't affect other end-to-end encrypted Apple offerings, such as iMessage, Keychain and Safari History. If you're in the UK and use iCloud, now would be a good time to switch to another cloud storage provider or use a local machine to back up your data. The process will be cumbersome and nowhere as seamless as it would be in Apple's ecosystem, you can rest easy knowing your data is secure.