You can't search content. This is the main reason i had trouble switching to ProtonMail in 3 years.
You can use the (beta) ProtonMail bridge to sync your emails into Thunderbird and search there.
I'm going to guess they implement something client-side using maybe IndexedDB or something?
Had the same idea when I read the comment. However, training your ML models in the browser is probably not the best thing you could do. Granted, it could potentially offer a more personalized service.
Search doesn't really need ML to be effective
If you search for travel, and the SE brings up all your reservations from flights, ferries, bus, etc. It requires some sort of ML. Stupid example but you get the point.
IIRC, you can do search with some homeomorphic encryption schemes, in theory.
In practice, yea, no search.
Yeah, but last time I checked homomorphic encryption was very very slow.