I juat saw that there is a Device Tree updated with OOS12 Firmware which can be used to build TWRP and I guess from that it should be absoloutly not so difficult to adjust that to work on our Oneplus 8t.
https://github.com/ApexLegend007/twrp_device_oneplus_lemonades
https://github.com/ApexLegend007/twrp_device_oneplus_lemonades/releases/tag/V1.0
It already works on 8T