Here's from 2020: https://web.archive.org/web/20201027235952/https://developer...

> Use text, not icons, for menu titles. Only menu bar extras use icons to represent menus. See Menu Bar Extras. It’s also not acceptable to use a mixture of text and icons in menu titles.

> Avoid using custom symbols in menus. People are familiar with the standard symbols. Using nonstandard symbols introduces visual clutter and could confuse the user.

The notable thing here is how recent of a shift this is, and how longstanding the prior rule was. Navigating internet archive is slow/tedious, but I think the rule/guideline was explicitly called out in the guidelines up until a year or two ago. So it was probably the guideline for ~20 years on macOS and has just now been changed.