When I was finishing up a bioinformatics doctoral program in 2009 I looked far and wide for postdoc positions in the field. My recollection is that the typical salary range was $27k-$30k. What surprised me was that I dug far back in a systematics-specific newsgroup and found the same salary range being advertised for the previous 10+ years. I can't speak to what has happened over the past 12 years, since the low postdoc salaries pushed me to industry, but I can say that you're off with the "more than two decades" claim, at least as a universal claim.