Now, the latest information suggests that the next update for Redmi Note 9 series such as Redmi Note 9 global, Note 9 Pro, Note 9S, and Note 9 Pro Max could be Android 11 based MIUI 12.5.
In addition, a forum moderator also stated that MIUI 12.5 update for Redmi Note 9 pro is under development. Before the MIUI 12.5 update, the company may release a system security patch.
However, Xiaomi has scheduled to release the MIUI 12.5 stable update in April. It is possible that Xiaomi will push the Android 10-based security patch for Redmi 9 series before the Android 11 arrives.