Does a rain repellent stop ice from forming on the windshield?

It helps, although it cannot fully prevent ice. Because water and frost bond less strongly to treated glass, ice tends to form a little later and is usually easier to scrape or melt off. In freezing conditions a frosted windshield will still need defrosting or scraping. Use a plastic scraper rather than metal tools, and avoid pouring hot water on cold glass, which can crack it. Scraping and de-icer fluids gradually wear the treatment, so it may need reapplying sooner after a harsh winter.