Since this is a retrospective, you can't easily find the exact 2021 flash build anymore (RIP Unity Web Player), but here is how we did it back in the day:
The specific keyword phrase was used by thousands of students who had memorized that the "911" domain (e.g., sites.google.com/site/unblockedgames911 ) reliably hosted a working, lag-free version of Slope. slope unblocked game 911 2021