Jet was a ubiquitous colourmap that slowly fell out of fashion over the last five years. It was the default colourmap for Matlab and Matplotlib (and probably others) until 2015ish. The replacements, Parula and Viridis, respectively, get a lot of love. A quick Google search will present numerous blog posts and articles maligning Jet and promoting the use of these newer colourmaps. It’s an unpopular opinion but, especially now that they are common and overused, I dislike Parula and Viridis as much as I dislike Jet.