I don’t see how a topper stops the rain from wetting any slide, so you have a wet slide anyway.
And the water from the topper has to go somewhere so the water will still be running down the slide. So topper or not you have to wait until its stops raining then dry the slides off, I never pull the slides in if they are wet.
I do notice that the windows on the side of the slides get dirty from the dirt on the top, when its a light rain. So out comes the window cleaner.