miui does not support the ability to set charge to 80% power off.
MIUI (Miyou) is a third-party mobile phone operating system deeply optimized, customized and developed by Xiaomi based on the Android[Android] system, and is also the first product of Xiaomi. Since its first private beta release on August 16, 2010, MIUI has reached 529 million users worldwide, covering 80 languages and 221 countries.
MIUI system Settings should not have this function, you can rush to 80%, manually unplug the charger