Yes, 'disused:' could be close to 'disabled:'. But 'was:' is clearly wrong.
'disabled' only refers to disabled people when used as a key (at least on https://wiki.openstreetmap.org/wiki/Key:disabled), not as a key prefix.
Still 'disabled:' and 'disused:' may have slightly different meaning. You again wrongly mean 'lights are not used ANY MORE'. But in reality, the lights are 'not used YET'. In such case prefixes like 'proposed:' or 'construction:' could be handy, but they again do not convey the information that the lights are already fully built, just not enabled/used.