> all of the Long Line towers I’ve seen in person (like the Slater station) have sat abandoned since the early 1990s.

I know of a few in Idaho that were operational and maintained at least up until the 2010s ish.

The one I'm familiar with was for a ranger station and a scout camp tucked away in the mountains. It was only replaced because of infrastructure spending for rural telecom which happened around 08 crash. All the sudden for these rural telecoms it made sense to put in a line for miles up a mountain for 1 or 2 people.