It is actively rolled out right now all the way back to iPhone 11 (2019) while the device is powered off. Version 16 is current, and the power “off” tracking was backported to older devices.
Android support is spottier. We’ve had powered off features one OnePlus for some time, such as the ability to trigger alarms while turned off, but more advanced features like location tracking are much more recent to Android because it usually requires specific hardware support to operate while using almost no battery. Apple has the privilege of vertical integration, so they were able to update older firmware.
I think this trend is very concerning, because with no user-servicable battery, we’re essentially forced into having our phones on to some degree at all times.