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.

1 - https://baymard.com/blog/line-length-readability

2 - https://www.w3.org/WAI/WCAG21/Understanding/visual-presentat...