Final answer:
PowerShell is pre-installed but not enabled by default on Windows Nano Server, which is designed to have a minimalistic footprint. It is available for but not installed by default on Linux systems and macOS.
Step-by-step explanation:
The question of which systems have PowerShell installed by default but not enabled is specific to Windows Nano Server. PowerShell is an automation and configuration management framework from Microsoft, and it is a command-line shell and scripting language, primarily used for system administration. By default, PowerShell is included in Windows Server and Windows operating systems.
For Linux systems and macOS, PowerShell is available but not installed by default and can be enabled if required. Windows Nano Server, which is an installation option for Windows Server, is a headless version with no GUI and has PowerShell, but it isn't enabled by default to maintain a small footprint.