Have they made any progress on this?

There should be two options/settings:
1. Option to set encryption remainder for all external mails.
2. Option to set encryption for specific users (I see this very well implemented in the contacts - There it can be a check mark to always request encrypted communication for that contact.

I consider it a critical functionality too.

I see that this is "Planned" - but the first suggestion was in 2016! How long can it take for what must be a very simple modification?
As Protonmail is aimed for security, surely a default or a setting option is essential

Like the other commenter (from way back in 2018), I also use Protonmail as a secondary account and infrequently. Too many times now I've failed to press the encrypt button. A setting that allows for a "Do you want to encrypt" prompt when you send an email would help solve this common mistake.

Completely agree. Once an encrypted conversation has started with an outside party any further message should be encrypted by default. As soon as you forget encrypting the message when you reply, the entire history of the conversation becomes visible in the recipients non-encrypted inbox.

I'm a bit worried that this has been an issue since 2015 (and hasn't changed yet).

Make CLEAR when a email to non-ProtonMail users IS encrypted!!! nOW IT IS completely unclear if sent message was encr. yes or no! The colourscheme is useless!
Besides you cant control afterwards if sent message was encr. Crap.

The "encrypt" button is quite far from the "send" button and is not very conspicuous, it is too easy to forget to encrypt the message. Since most of your recipients won't have protonmail anyway, not encrypting a mail by default sort of defeats the purpose of protonmail.

This would be a very helpful feature. I use protonmail as a secondary account for sensitive data, etc and always encrypt external messages, but I have worried I will forget to choose the option as it is such a small button at the bottom of the compose mail window.

Option to have all outgoing email encrypted by default would be huge!

It would be nice to have a setting "Prompt for Encryption" that alerted the user before an unencrypted message was sent to a user outside of ProtonMail. This would prevent accidentally sending sensitive information without it first being encrypted.

I noted this when a draft, which I had previously setup for encryption, was sent unencrypted when I loaded the draft and sent it without double-checking.

Perhaps a more intuitive way could be to show a green lock when the communication is secure vs a red (open lock) to remind you to encrypt your communication. Similar to how browsers act when using SSL vs plaintext. Most people recognize a red open lock, and understands what that means.

This is a MUST. Even if highlighting a non-protonmail address in bright red after it is entered.

Tutanota puts a big colored button under the subject to make it very visible whether outgoing mail is encrypted or not. Also, there is an option to encrypt ALL outgoing mail by default. Implementing this feature will prevent a lot of mistakes by accidentally sending mail in plaintext.

YES!
I feel this is a critical feature for a secure system. I just inadvertently sent a message without turning on encryption. Fortunately this was a test, but if it had been real ...OMG.