Oneplus lauched its OnePlus 8T and 9R handset last year in March with the LPDDR4X RAM.Since then the handset hardly go any upgradation until now.
The latest OnePlus 8T and 9R units were spotted with the faster LPDRR5 RAM. But the compnay hasnt yet made this official, with no chages in the listing page for the specs for these handset.
While its not clear why the handsets were upgraded to the LPDDR5 RAM, you can check this on your handset by using the app Temux and run the command getprop ro.boot.ddr_type. If the value returned is 1, then your device has the upgrades LPDDR5 RAM.
via - GSMArena