After thinking about this discussion for a while, I tend to agree with you an elegant solution would be that ‘user’ is an extension (requiring an email address) available to any type, which adds the behaviours (permissions, notifications etc.).
However, I saw other threads that Polina acknowleged that (free) read-only users were in the pipeline: [DONE] Read-only guest users? - #3 by Polina_Zenevich
Virtual or Dummy User - #4 by Polina_Zenevich
so perhaps that will be good enough.
I note that Polina also said that until they’re implemented Fibery might be able to help with some free accounts if it was an important need.
Finally, FWIW you can create inactive users and they can be #mentioned (but can’t be linked to unfortunately).