While I do realize that most of the neurological symptoms were tosh, or easily attributable to the high alcohol content, just play along on this train of thought for a minute:
Distillers had different recipes for absinthe. Some of them included Chinese star anise (Illicium verum). Macromorphologically, Chinese and Japanese star anise are practically identical. Fluorescent microscopy and scanning electron microscopy are the only way without resorting to chemical fingerprinting to tell the difference between the two (None of which was available back then). Even today there are instances of tea recalls and such because of adulteration. What if a distiller who didn't know any better, couldn't tell the difference and used the wrong Illicium species in his recipe? What if maybe a handful of cases of neurological symptoms weren't rubbish? What if people merely blamed the wrong plant all this time?
I know it's a huge 'what if', but it's something that caught my attention. The main sesquiterpene lactone, anisatin, effects the same neurotoxic symptoms as thujone.
'Anisatin is quite stable in alcoholic solutions under ordinary conditions.' It is recognized as a convulsant. (Kudo, Y., Oka, J. I., & Yamada, K. (1981). Anisatin, a potent GABA antagonist, isolated from Illicium anisatum. Neuroscience letters, 25(1), 83-88.)
Actually I. anisatum is a neurotoxic plant because it contains sesquiterpenic lactones. (Kim, J. Y., Kim, S. S., Oh, T. H., Baik, J. S., Song, G., Lee, N. H., & Hyun, C. G. (2009). Chemical composition, antioxidant, anti-elastase, and anti-inflammatory activities of Illicium anisatum essential oil. Acta pharmaceutica, 59(3), 289-300.)
Botanical ingredients of absinthe and the different recipes are part of what I'm looking into in my research. What if some idiot looking to make a quick buck producing cheap absinthe thought Japanese star anise would be a good ingredient? Like I said, it's a bit of a stretch, but what if?