It is generally stated, although it does not appear to be true.
Err - no. The (now very old) USGS report on Pinatubo includes several notes about aircraft damage found well AFTER flights that could have caused it! The authors commented that the reason that there's no location given for some of the tabulated incidents is that no-one knew where they happened!
And I was not suggesting more studies - there already seems to have been plenty of evidence, even from the early 90s. I meant real-world policies and procedures that actually MEASURED stuff (like ash in the air and collected on / in aircraft when there was likely to be a problem) and then ACTIONS to minimise risk and further cost. Ad hoc Mk 1 Eyeballing ain't it!