Re:Protonmail can delete the wrong email and nobody cares
by timtom39 on Wednesday November 02, 2022 @10:45AM
(#33436649)
bridge exposes imap making it easy to download all your data using Thunderbird or another client. I don't know about "export" because imap does what I want and is supported by most providers in the same format.
Here: https://github.com/ProtonMail/proton-bridge
Briefly looking at the files and code it's hard to tell whether that is still the case, but it's fair to assume Import-Export would reuse most of the machinery behind Bridge.