More information about Xiaomi's memory extension feature has been shared, including additional eligible devices and how much extra RAM can be accessed by specific phones. It appears the Redmi Note 8, POCO X3 GT, and Redmi 9T have joined the eligible device list, which will likely also feature the Xiaomi Mi Mix 4 very soon.
https://www.notebookcheck.net/New-Xiaomi-memory-extension-details-add-Redmi-Note-8-and-POCO-X3-GT-to-the-eligible-device-list-and-reveal-how-much-extra-RAM-can-be-utilized.554398.0.html