This code is creating 6,001 line objects which each have a single vertex. A line object with a single vertex is a degenerate case which isn't guaranteed to draw anything. Different renderers in different versions of MATLAB have handled this case differently, so this has never been a very reliable way to plot.
One option is to tell plot that you're drawing dots instead of lines. That's as easy as changing your call to plot to look like this:
But creating 6,001 objects which each draw a single dot isn't a very efficient way to go, for this reasons I explained in this post on the MATLAB Graphics blog . You'd really be better off with a single call to plot. In your case, it would look something like this.
t = 0:dt:6;
h = zeros(size(t));
v = u + (g*dt);
dh = 0.5*(u+v)*dt;
h1 = prev_h-dh;
h(i) = h1;
prev_h = h(i);
u = v;
The other advantage of this approach is that you could draw lines connecting the points instead of individual dots. With your current approach you can't do that because the plot command can't "see" more than a single data value at a time. By passing all of the data values into plot together, it can connect them.