Maxon no longer offers the R21 demo, but if you have a friend who downloaded the official trial installer back in 2020, that file still works for a 14-day, fully functional trial.
Maxon offers a 42-day free trial of the full Cinema 4D subscription (currently R26/R2024+). While not R21 specifically, it’s far more capable. You can: cinema 4d r21 free download exclusive
If you have a .edu email address, apply for Maxon’s Education License. You’ll get the latest version free for 6 months (renewable). Many schools also provide access to older versions like R21 for course continuity. Maxon no longer offers the R21 demo, but