The documented properties are at https://www.mathworks.com/help/matlab/ref/matlab.graphics.illustration.legend-properties.html
For undocumented properties,
h = legend();
s = struct(h);
This will list a number of data classes that have no public description like [1×1 ListOfPointsHighlight] or [1×1 UniformCartesianDataSpace] . Some of those turn out to be linked to implementations that Don't Work Right.... for example setting the Alpha component of a LineStrip color should work (the java objects do have alpha) but doesn't.
