Unvolting or underclocking means reducing clock speed to increase battery life. This is done using a custom kernel that allows you to do exactly that and it is now available for the Samsung Galaxy S21 series, including the base S21, S21 Plus and S21 Ultra.
How to install custom kernel on S21?
go to your custom recovery like TWRP (for s21+) – for S21 ultra, get this twrp.
Download and install the zip
Reboot and that is it.
Features of this custom kernel.
upstreamed to linux 5.4.294
Based on last source kernel for Linux 5.4
Built with the latest AOSP Clang 20.
Kernel SU Next support.
SUSFS Support.
Undervolt.
Optimized for Unity-based games.
Reduced GPU thermal throttling.
EROFS backported from linux 5.10
It is compatible with any One UI based ROM. If you have a rooted S21, flash a non-root version and then flash magisk to get root access again. You will any warranty if you have left any on your S21, so keep that in mind before flashing this custom kernel on S21.