Ruler Issue in Illustratror CC 2020

"There is a new checkbox in Preferences > General: Show/Hide Rulers Across All Documents."

Yes, this makes me very happy, although the implementation may seem less than straightforward at first.

With the new checkbox un-checked, rulers remain document-specific.

With the new checkbox checked, both Show and Hide (hence: 'Show/Hide') take effect "Across All Documents". That is, if you look and don't see the rulers, you are in the Hide Rulers Across all Documents state, and if the rulers are visible, you're in the Show Rulers Across all Documents state.

I think you may get what you want in the 2020 version of Illustrator.

There is a new checkbox in Preferences > General: Show/Hide Rulers Across All Documents.

This was not an option in previous versions, but exactly what you want.

I understood your question.

What you describe (open a document, show rulers, open a new document and the rulers appear also in the new document), is true for Photoshop, but not for Illustrator.

Rulers are part of the document preset.

The show/hide ruler is part of the new document profile that you choose when you create a document.

https://helpx.adobe.com/illustrator/kb/create-custom-new-document-profile.html