Edit: It is now available in UK. 🙌🏻
A large software update has started rolling out in India this morning for the S21 series ( Monday 29th March 2021 ) it is around 1GB in size with April 2021 patch and firmware version G99xxXXU2AUC8. Camera improvements, overall performance improvements. Changes to portrait mode.
I haven't received it myself so far, however it will be available in other countries very soon.
I am in the UK with S21 Ultra SM-G998B - BTU.
My device.
India device.