We bought our Xiaomi Mi Box S a few months ago and I was disappointed cause a month later, The Mi Stick 4K was announced and it has a newer version of Android TV which is Android TV 11 and has support for Dolby Vision and Dolby Atmos. But it was fine, it's just I feel like Xiaomi doesn't care about the 2018 Mi Box S anymore even tho the Mi Box S is still on Xiaomi's Website in the TV section. The Xiaomi Mi Box S is still running on Android TV 9 originally running Android TV 8.1.
Mi Box S last got an update on October 2021 (along with the 5 year old Mi Box 3), so I wouldn't rule out a future update yet.