For some reason that we don't know yet , the Spot Slope annotation tool does not work on ramps in plan views. It works on elevation, sections, and 3d views, though; however, it's on the plans where I need it most. Until this is fixed in the future, here's a solution that you might find useful:

From a 3d view, create a model line with thin line style, aligned and locked to one of the slanted edges of the ramp. (That line is represented in heavy red in the image, for clarity). Then go to the plan view, and try again the Spot Slope annotation tool on the ramp, but this time use the tab key to find the model line. Now you have the slope of the ramp.

If the slope of the ramp changes, the annotation will update, since the line is locked to the slope of the ramp. The ramp can be edited, shortened, lengthened, and moved, without affecting the slope annotation at all. Only issue I found is that when the ramp is rotated, the constraint is lost, but then it is very easy to align and lock the line to the ramp again, and the Slope annotation goes back in place again, updated.