I remember it being the only thing on Asplundh's old website.
Now that you mentioned it, it may be a directive coming from Altec. When Altec bought Asplundh, Altec's legal department suggested they that Altec limit their liability by getting rid of all the Asplundh equipment they could. So when Asplundh Tree trades in old drum chippers for new Altec chippers, Altec has them cut up. All the parts build by Asplundh go the scrap. Perfectly good chippers all gone, 1000's of them. Those under contract to do this can not even resell the parts, not the parts the Asplundh build anyway.
So maybe that bulletin went out from Altec and is not related to an actual problem with the booms but is just Altec reducing their liability.
I have known company that like those old booms and keep them working themselves. It is important to keep the cables in good condition and base free of cracks. The problem is if the boom fails and there are injuries the owner would be criminally negligent.