Proton Mail can give email content, however, it is encrypted and they do not have the encryption keys.
Anything that is stored by anyone can be handed over. That information may be useful, may be useless or may be useless now and useful tomorrow when they have the key.
> True, but they can trivially obtain them given they control everything in the browser.
Open source clients that you can self-host are available. I mean of course you still have to trust the code if you can't audit it. But hijacking your keys won't be as easy as visiting their webmail.
Anything that is stored by anyone can be handed over. That information may be useful, may be useless or may be useless now and useful tomorrow when they have the key.