Here’s the NOTAM for the slack line you mentioned, it was posted on the FAA website before that helicopter accident happened, and it shows that the line was marked with aviation flags and strobes (“LGTD”). Too bad the pilot didn’t see the NOTAM...
They are already online for all to see - pilots and anyone doing anything in the airspace should be reading them before flying, including all of us before we fly a drone. Here’s the link:
https://notams.aim.faa.gov/notamSearch/