In opposite to what happened in 4.x, it doesn't seem possible to redefined the id for user anonymous in a 5.x setup.
In 4.x I was able to create a new anonymous user, delete the default one (id:10) and defined my new one in site.ini.
In 5.x that change is not possible, yet.
Steps to reproduce:
- create a new user under anonymous user group
- include the [UserSettings]AnonymousUserID=<new-id> in override/site.ini
- delete the default anonymous user
=> Both the back end and the front end will cease to work