When did this notification happen and by which channel?

It was discussed in the media when Apple announced that UK users could no longer enable ADP if they had not done so already.

> For users in the UK who have already enabled Advanced Data Protection, Apple will soon provide additional guidance. Apple cannot disable ADP automatically for these users. Instead, UK users will be given a period of time to disable the feature themselves to keep using their iCloud account.

https://support.apple.com/en-gb/122234

Right, I saw that. That’s not as explicit as what you claimed, though:

> They notified users that they would lose access to their encrypted data in the future

There is an implicit threat of data loss here but it’s far from clear.

As I mentioned, it was discussed in the media back in February.

> According to Apple’s Xcode console logs reviewed by BBC News, existing ADP subscribers will lose access through phased certificate revocation in Q2 2025.

https://cybersecuritynews.com/apple-discontinued-adp-feature...

The UK publicly backed down before that happened.