Proton Mail/Drive/Calendar should support Siri AI integration with this fall's release of iOS and macOS 27. Having thoroughly researched Apple's new Foundation models and AI architecture, its clear that they are prioritizing use of on-device models for the vast majority of requests and only going to Private Cloud Compute (PCC) if needed (in which data is deleted right away after the request is fulfilled). You can independently verify Apple's PCC claims here https://security.apple.com/documentation/private-cloud-compute/verifiabletransparency.

Having tested these AI features myself, they make my life significantly easier with being able to find the things I need just by asking or typing and I think this will be a must have for me to continue to use Proton as my primary email and cloud storage provider. If possible having the features be opt-in would be completly acceptable in my opinion.