It would be great if Matlab documentation included a list of built-in data sets. This question has been asked before (2012, 2013, 2014, on statckoverflow) with at least 2200 views in the past 30 days but still I cannot find such a list.
The census MAT-file in toolbox/matlab/demos (used here among other places) seems like it's obviously a data set. So does outages.csv (used here.)
Are peppers.png and ngc6543a.jpg (used here) also data sets? They're image data, "peppers" appearing on 38 pages in the documentation for MATLAB and 61 times in Image Processing Toolbox. "ngc6543a" occurs less frequently, but still on 19 pages in the documentation for MATLAB.
What about the video file shuttle.avi (used here?)
Does the peaks function (often used in examples to create a simple piece of data to visualize as a surface plot) count as data or code? What about the functions that generate the predefined colormaps or the gallery function?
You can submit an enhancement request to Technical Support asking for such a list to be added to the documentation for one or more products. It would be useful to include in that enhancement request what constitutes a data set in your mind and what information about each data set you'd expect to see in such a list.
I started a list of built-in mat files and a brief description of their variables (r2019b). Since numeric variables are fairly easy to create on the fly, I only listed files that contain other classes of variables for now. Anyone with edit privileges should feel free to add to this list.