It's been this way forever because it's not particularly difficult science and is extremely easy to test for so there are probably thousands of papers covering this. Here's a good summary by Baymard Institute[1].
Also WCAG recommends line length set to <80 characters too [2]. I'm not sure what else could make this more convincing or official.
Just like lelanthra currently applies custom CSS `body { max-width: 38rem; }` to this page, if the page had that maximum width set by default, you would equally have the ability to apply the CSS `body { max-width: unset; }` to the page. So you would not be denied the option of longer lines.
Also WCAG recommends line length set to <80 characters too [2]. I'm not sure what else could make this more convincing or official.
1 - https://baymard.com/blog/line-length-readability
2 - https://www.w3.org/WAI/WCAG21/Understanding/visual-presentat...