If it’s just brown plaster it just comes off with a wet cloth the things to avoid are cement splattered windows,I have used razors in the past and still use 1 for labels and celotape only in 6” form.
No matter how careful you are with scrapers there’s a very very good chance you’ll scratch the glass with them especially if you do a big area with say a 2” scraper,the best thing to remove any debris from glass is Bronze wool I’ve used it the last few years and it’s miracle stuff 95% of the time now a scraper is redundant.