This is good advice, because email is very difficult to make reliably private. However, it’s not the best you can get. Tutanota, for example, stores headers with E2EE, and still has a search function.
The goal should be to make it as private as it can realistically be. Ideally, any cloud service you use should only store end-to-end encrypted data.
I’m not trying to shit on Proton — it’s a huge step up from the popular mainstream email services, and the inclusion of cloud storage makes it a much easier transition than going piecemeal with 2-5 different services.
I feel bad for this kid. That really is a bad warning dialog. Nowhere does it say it’s going to delete files. Anyone who thinks that’s good design needs a break.
Half the replies are basically “This should be obvious if your past five years of life experience is similar to mine, and if it isn’t then get fucked.” Just adding insult to injury.