When Advanced Data Protection is enabled, access to your data via iCloud.com is disabled by default. Users can turn on data access on iCloud.com, which allows the web browser and Apple to have temporary access to data-specific encryption keys.
This makes me question if I should turn it on or not. I have viewed iCloud.com as a means to get at some of my data in a pinch (say I’m on vacation and break my phone and need to get some info off a computer in a hotel’s business center). It sounds like that would no longer be possible.