The Full Access role has been inactivated for all users. The Full Access role no longer appears in the list of roles available for users who had this role assigned.
The Core Administration Permissions checkbox is available on the role configuration page on Setup > Users/Roles > Manage Role and edit one role. This is created for the alternative of Full Access role.
The Core Administration Permissions can be assigned to any role as an extension to standard permission settings. With Core Administration Permissions, the role will get access to most of the functionality that was previously only available to the Full Access role.
Use caution when assigning Core Administration Permissions to a role, because the role will become similar to the standard Administrator role in terms of exclusive administrator privileges.