You keep banging on about checking filters on engine bleeds.
Mind enlightening us
The NASA report on the DC-8 incident stated specifically that ash was found in the filters of the cabin AC heat exchangers. (The filters were changed at Kiruna on the assumption that ash WOULD be trapped but actual examination apparently waited until after the aircraft got back to Edwards AFB.)
I assumed (incorrectly??) that if the DC-8 design required such filters, they'd be fitted on later aircraft too.