Everyone except external users disappears form Group Visitors

Martin-Coupal I think you should join the discussion on GitHub or add feedback on the document in question. The docs are opensource now, so anyone can contribute. (I work on the freelance team that helps to resolve issues on GitHub and I saw this discussion while trying to figure out a change that mentioned this, but did not explain it very well). You can open a new issue on the content page that should be changed - perhaps this one: https://docs.microsoft.com/en-us/sharepoint/understanding-permission-levels (scroll to the bottom and add your feedback to "this page" to open an issue on GitHub) is where to request that this be documented.