encrypted email

Besides being a form of messaging (so the text somewhat contradicts itself), typical email is a deeply insecure protocol.
In my opinion, it's probably impossible to secure without making a new protocol or making such drastic changes that it might as well be considered one.

Here are some key concerns regarding the usual PGP-powered encrypted email:

• Email, at a simple level, works much akin to physical email — there's an "envelope" containing important info regarding the communicating parties, which can't be encrypted, otherwise the mailing servers wouldn't know where to forward the messages. This essentially leaks a lot of metadata that can be almost as valuable as the message body itself.
• There's no forward secrecy — one of the best cryptography features that has become pretty much a commodity in modern systems is forward secrecy, which prevents attackers from decrypting older messages after gaining access to one of the keys.
• While not an issue with the protocol itself, it's the sad reality and we need to consider — most people use GMail, Outlook and the like, which ultimately need to read your emails in plaintext, for better or worse reasons (search is incredibly useful, but some big players don't stop there of course :p).
• Another thing is the fact that it's incredibly easy to have an imbalance of encryption, i.e. someone is encrypting their messages, but others aren't. With the very popular email culture of quoting (be it top or bottom posting), an unencrypted party in the the conversation can leak important information.
• PGP is... peculiar, so to speak. I has a lot of issues, mostly stemming from its age (which could also be a source of robustness and security, due to being very battle-tested, but I don't think that's quite the case with PGP/GPG), tries to do too much and typically has a clunky UI, which impedes wider and proper adoption by less technically people.

This isn't to say people should definitely stop using and promoting encrypted email, since it can be useful.
It's just it gives, more often than not, a false sense of security and can lead less proficient users to send sensitive data through this medium which isn't nearly secure enough for such use cases. Preferably, people with such threat models should opt for better alternatives, most suggested in that article (such as, but definitely not limited to, Signal, SimpleX, Matrix+Olm, XMPP+OTR/OMEMO, sharing files via MagicWormhole, encrypting with tools like age).

On a slightly tangential note, I think someone should make a Matrix client with an email client interface. I started working on a new traditional chat client (completely nonfunctional still, very much in-dev), but I've been honestly thinking more and more about making one looking like an e-mail client, where there isn't much focus on instant room-based chats, but rather on longer-lived 1-to-1 and list-like exchange of messages.