In the Classic editor, there's the Help section with a (limited) user guide about:

• editing the title and the post
• inserting media
• publish settings
• discussion settings

there's also a modal with a list of all the available keyboard shortcuts:

To my knowledge, in Gutenberg there's no help or user guide at all, not sure if this was discussed at some point. I'd strongly recommend to start thinking at a user guide. Whether it lives in the admin or on an external page / handbook, it should be easily discoverable.

As per keyboard and screen reader users, they need a well documented list of all the available keyboard shortcuts in Gutenberg, plus a guide to some of the available accessibility features, e.g. the Ctrl + backtick shortcut to navigate through the main editor regions.

We've discussed this during today's accessibility meeting and agreed the list of keyboard shortcuts is essential to effectively use the editor and should be available since version 1.