With Raspberry 4, I used the SLC pin together with myGPIOd to create an On/Off switch, but Raspberry 5 ships with a power button and you can add an external Button to the J2 jumper.

This is described in the Raspberry documentation.

You can enable the button easily in DietPi:

• Set HandlePowerKey=poweroff in /etc/systemd/logind.conf

• Make sure that ACPI, DBus is installed and activate systemd-logind:

  apt install acpi dbus
  systemctl unmask systemd-logind
  systemctl start systemd-logind