The behavior of dimming out a legend entry if its respective line's visibility is turned off was changed in R2016a. Currently, there is no way of disabling this behavior. I apologize for the inconvenience.
As a workaround, you could plot using NaN's so that the legend entry is not grayed out. This is depicted in the following example:
The code below will have the legend entry grayed out.
l = line(1,1)
l.Marker = 'o'
l.Visible = 'off'
Plotting with NaN's will avoid that behavior:
l = line(nan,nan,'Marker','o');
This way, the line is not shown, but it is still treated as visible so the legend entry is not grayed out.
I hope this information was helpful.
Have a nice day!