There’s no regulation on content of the post, so you can encrypt your message, print it and send it by post. Equivalent of the court order in digital world is the permission to obtain whatever version of the content is available.

Mandating that all mail should be written in such a way that someone from the government could understand it, is clear overreach.

I agree with the argument on the logical level, but in practice I don't think it should be used be used, at least not as the first argument. For the general public, talking about encrypting a physical letter makes you look even more paranoid / malicious than when talking about online encryption.

It’s HN, the public here is slightly more educated in digital affairs, right?

In practice there are physical limits on how much phonecalls or snail mails can be improperly publicized.

Online even the stuff that very rich companies struggle very hard to keep private regularly gets publicized in bulk.

You might think in terms of "medium is the message" so you can't directly transfer something that works in principle for one medium to another.

If they get a court order then they can start trying to break the encryption.

But you're not obligated to make it easy for law enforcement to enforce any potential court orders, though. It's perfectly legal to install a safe that is impossible to open without destroying its contents, for example.