The "+1" in your search likely refers to the film being the first entry in the Taboo series. The original 1980 film was followed by numerous sequels, including Taboo II (1982), Taboo III (1984), etc. So Taboo (1980) = Taboo 1 in the franchise.
The film’s lasting reputation is largely attributed to its primary cast and surprisingly high production values for the time: taboo+1+1980+imdb