Feature flags are now empty but I was able to turn it on using adb.
The settings are:
Turning it on:
adb shell settings put global settings_screenrecord_long_press true
Turning it off:
adb shell settings delete global settings_screenrecord_long_press
To use it long press on the screenshot button
The settings are:
Turning it on:
adb shell settings put global settings_screenrecord_long_press true
Turning it off:
adb shell settings delete global settings_screenrecord_long_press
To use it long press on the screenshot button