At the MIUI 9 Global launch, Xiaomi highlighted the fact that even 5-year-old phones including Mi 2/2S will be supported for MIUI 9 Global Stable update but lately things are changing, probably due to many devices launched in past couple of years.
A few months ago, the company announced that it has decided to drop MIUI development for few devices including Mi 5, Mi 4, Mi 4S, Mi 4C and Redmi Note 3 Pro (called Redmi Note 3 in India), now as per a post on MIUI Forum, Xiaomi has decided to suspend MIUI ROM (Global and China both) development for Mi Max and Mi Max Prime.
Post from MIUI Forum admin says,
Due to the comprehensive factors such as device release date, device activity & stability and software experience, MIUI has released the last development version of 9.2.28 on March 1, 2019, which is the last development version of Xiaomi Max and Xiaomi Max Prime. These two devices will no longer support subsequent version upgrades. At the same time, we have also prepared a stable version of the upgrade program, Mi fans can keep the development version or schedule an upgrade to the subsequent MIUI 10.3 stable version (expected to be released in April).
Xiaomi has suspended MIUI Beta ROM development with immediate effect from 1st March saying that MIUI Global Beta 9.2.28 is the update for Mi Max and Mi Max Prime. Also, MIUI 10.3 will be the last Stable ROM release for Mi Max and Mi Max Prime which is expected to be released in April. Xiaomi has recently introduced dark mode on MIUI Global Beta ROM and it has already kicked off MIUI 11 development but sadly Mi Max, Mi Max Prime, Mi 4, Mi 5 and Redmi Note 3 Pro (called Redmi Note 3 in India) will not receive MIUI 11.