It's quite hard to find examples of good mapping guidelines online - not just the "how to" or generic advice, but comprehensive guidlines about a set of styles an organisation uses.
The Economist does share theirs online through their design system subsite: The Economist Maps style guide.
They're pretty good at sharing these kind of documents, perhaps related to the fact that they sell copies of their writing style guide as a standalone product.
The guide has sections on:
- Text styles
- Lines and objects
- Colour palettes
- Colour scales
- Locator maps, country-level maps and street maps
- Map projections
I'd love it if more organisations shared this kind of material from their design systems. Like the Economist, the Guardian and the BBC also share their writing style guides online but both are missing any material on maps.