I remember this from when Siri was first released. It was only available on the 4S if I remember right, and with a jailbroken device you could backport it since most of the assets and functionality was already there - sans the authentication keys.

It did not work well at all if you force enabled it on an unsupported device.

Or Always on Display, which usually only supports OLED capable devices could be force enabled on LCD devices, at the cost of your battery life.