Protonmail is cop friendly. Any reasonable privacy aware email provider would hash the secondary email, not store it as cleartext. I implemented secondary email hashing for Riseup to prevent exactly this thing, over a decade ago.

https://restoreprivacy.com/protonmail-discloses-user-data-leading-to-arrest-in-spain/

Proton Mail Discloses User Data Leading to Arrest in Spain

Proton Mail came under scrutiny for its role in a legal request by the Spanish authorities leading to the identification and arrest of a user.

RestorePrivacy

Eeeeeyikes.

Just when I switched to Proton.

@theothersparrow The name/address of the terrorism suspect was actually given to police by Apple, not Proton. The terror suspect added their real-life Apple email as an optional recovery address in Proton Mail. Proton can't decrypt data, but in terror cases Swiss courts can obtain recovery email.
@protonprivacy @theothersparrow calling Catalon independence movement "terrorism" is technically what the Spanish state says, but is completely laughable and destroys all meaning of the word. The point remains: the request "what account is associated with this email" is not a question Proton should be able to answer.