Since ROLE_GUEST and the guest user were superfluous - the default Symfony IS_AUTHENTICATED_ANONYMOUSLY & anon. user cover the same use case - we decided to drop them in favor of the Symfony defaults. This is a BC breaking change, so a migration path is provided.
To upgrade, first pull in the new version of all bundles.
Remove guest_user: true from app/config/security.yml.
And finally execute the following command :
bin/console kuma:fix:guest
This should execute the necessary changes (you could delete/rename the guest user afterwards as well - we just leave it in case there are items linked to it).
Note: it will no longer be possible to add extra roles to the guest/anonymous user in the back-end. You will also have to adapt your code (in most cases replacing ROLE_GUEST with IS_AUTHENTICATED_ANONYMOUSLY should suffice though).