Redmi 3S Prime, purchased on 31-Aug-2016, haven't been receiving any updates since this thread was posted. It is running MIUI 10.2.2.0 on Android 6 Marshmallow. Android Security Patch is April'2017.
A 3GB/32GB phone with 128GB SD Card and 4100 mAh battery, this device still runs strong and can be used for all purposes. I wish it got Android Pie update, but too much to expect I think. The phone is only 3 years old now.
Good news for Redmi 6 series users.
Redmi 6 and Redmi 6A users in India have reportedly started receiving the MIUI 10 update based on Android 9.0 Pie. The company, in April, revealed that it had suspended development of this update for the two phones. However, in June, the company changed its mind and announced that the two phones will indeed be upgraded to Android Pie. Redmi 6, Redmi 6A Getting Android Pie-Based MIUI 10 Update in India
Redmi has been quite considerate. Getting MIUI11 on a phone as old as Redmi Note 4 is awesome. I sometimes regret not waiting for Note4 and going for 3S Prime in Aug'2016.