The access content permission is now provided by the System module. This means it is always present and can be relied upon by all modules. Before Drupal 8.5.0 it was provided by the Node module but modules such as File used it even though they had no dependency on Node.
Module developers are not expected to have to make any changes. This change allows them to use the access content without relying on Node. An example of this is #2843139: EntityResource: Provide comprehensive test coverage for File entity, and tighten access control handler, which now requires users to have the access content permission to be allowed to view files. (Downloading files still requires no permission, so there's no real-world BC break there.)
For Site builders and administrators the position on the user permissions form has not changed if Node is installed.
