Actually, it's probably text/plain; format=flowed, which any sane email client would display as appropriately wrapped for the viewport size. It's generally the best way to send plain text emails.

The problem here is the mailing list archive viewer, which is displaying it in a non-wrapping context, instead of letting the text wrap appropriate. It's using a <pre> instead of a <pre style="white-space: pre-wrap">, in other words.

Why put in hard breaks when pretty much any email client should support soft/flexible line wrapping?

Seems more like an issue with the linked site to me.

Email clients that autowrap text.

Line breaks are hard.

But <format=flowed> has been causing problems for years. https://joeclark.org/ffaq.html