Can't comment on that specific case. But in a lot of cars it's just there purely so things cannot be powered up without the ecu being plugged in. No fancy control, just a direct earth path through the ecu to the relay.

On some more modern cars, it can be because the ecu can remain live for other things for a period after key off for example, so the ecu has a section that can allow this by controlling the power relay. I doubt that's the case with this old ecu though.

So you could check for grounds at all the other relevant pins. Generally inside an ecu, all power grounds will be linked so you could bell test them to see if this is the case.
As well as checking chassis/battery grounds.

ECU sensor grounds should not be linked to power grounds.