That is one of the older web superstitions/myths. If some folks chose to advertize that they are superstitious, it is probably more polite to not notice.
Writing out pleco, or even plecostomos, has never been a problem for me. Plecostomus plecostomus was once the scientific name of the "common" pleco, but that was revised to Hypostomus plecostomus, and now has been subdivided into several species. I'm not certain there is a fish with the real name "plecostomus" any more, so I am not sure what fish is supposed to die if you write pleco, or how this is supposed to happen.
Off-topic: Thanks for the email.