According to XDA, a Xiaomi spokesperson said that the company will be dropping the Mi name for future products. This officially started with its most recent release, the Mix 4. That leaves the Android overlay MIUI as the only thing left using the name.
Overall, it’s a decision that makes sense. Back in the day, the Mi name was useful to distinguish between the mainline series of devices, and the more affordable Redmi line. Said line has become its own dedicated sub-brand since the release of the Redmi Note 7 all the way back in early 2019. POCO also followed suit a year later. This makes the Mi name as a distinguishing element between the lines of devices less useful. All that being said, Xiaomi itself still has two lines of devices to work with, despite dropping the Mi name. One is the mainline series of devices which are just named after numbers now, and the Mix line, which the company says are the “concept device that users can buy”. (Source: XDA)