[OnePlus 10 Pro] g96818:
I'm not sure if you can flash the downgrade rom through recovery or not, but you can try the following as a last resort. I am not responsible if you end up breaking your phone and this should only be used as a guide that you need to modify yourself since this script came from a oneplus 9RT fastboot rom.

@echo off

ECHO ###################
ECHO # Formatting data #
ECHO ###################
fastboot format userdata
fastboot format metadata

@echo off
title OnePlus 9RT Fastboot ROM Installer by abhinavgupta371 and HELLBOY017
ECHO #######################################################################
ECHO # Oxygen OS Flasher For OnePlus 9RT by abhinavgupta371 and HELLBOY017 #
ECHO # updated by Va1d3m #
ECHO #######################################################################

ECHO #################################
ECHO # Flashing firmware into slot A #
ECHO #################################
fastboot --set-active=a

ECHO #################################
ECHO # Flashing bootloader img's #
ECHO #################################
fastboot flash --slot=all boot boot.img
fastboot flash --slot=all modem modem.img
fastboot flash vendor_boot vendor_boot.img
fastboot flash vbmeta vbmeta.img
fastboot flash vbmeta_system vbmeta_system.img
fastboot flash vbmeta_vendor vbmeta_vendor.img

ECHO ##########################
ECHO # Rebooting to fastbootd #
ECHO ##########################
fastboot reboot fastboot

ECHO #################################
ECHO # Flashing fastbootd img's #
ECHO #################################
fastboot flash --slot=all abl abl.img
fastboot flash --slot=all aop aop.img
fastboot flash --slot=all bluetooth bluetooth.img
fastboot flash --slot=all cpucp cpucp.img
fastboot flash --slot=all devcfg devcfg.img
fastboot flash --slot=all dsp dsp.img
fastboot flash --slot=all dtbo dtbo.img
fastboot flash --slot=all engineering_cdt engineering_cdt.img
fastboot flash --slot=all featenabler featenabler.img
fastboot flash --slot=all hyp hyp.img
fastboot flash --slot=all imagefv imagefv.img
fastboot flash --slot=all keymaster keymaster.img
fastboot flash --slot=all mdtp mdtp.img
fastboot flash --slot=all mdtpsecapp mdtpsecapp.img
fastboot flash --slot=all multiimgoem multiimgoem.img
fastboot flash --slot=all oplus_sec oplus_sec.img
fastboot flash --slot=all oplusstanvbk oplusstanvbk.img
fastboot flash --slot=all qupfw qupfw.img
fastboot flash --slot=all qweslicstore qweslicstore.img
fastboot flash --slot=all shrm shrm.img
fastboot flash --slot=all splash splash.img
fastboot flash --slot=all tz tz.img
fastboot flash --slot=all uefisecapp uefisecapp.img
fastboot flash --slot=all vm-bootsys vm-bootsys.img
fastboot flash --slot=all xbl xbl.img
fastboot flash --slot=all xbl_config xbl_config.img

ECHO ###############################
ECHO # Resizing logical partitions #
ECHO ###############################
fastboot delete-logical-partition odm_a
fastboot delete-logical-partition odm_b
fastboot delete-logical-partition system_a
fastboot delete-logical-partition system_b
fastboot delete-logical-partition system_ext_a
fastboot delete-logical-partition system_ext_b
fastboot delete-logical-partition product_a
fastboot delete-logical-partition product_b
fastboot delete-logical-partition vendor_a
fastboot delete-logical-partition vendor_b
fastboot delete-logical-partition my_carrier_a
fastboot delete-logical-partition my_carrier_b
fastboot delete-logical-partition my_company_a
fastboot delete-logical-partition my_company_b
fastboot delete-logical-partition my_engineering_a
fastboot delete-logical-partition my_engineering_b
fastboot delete-logical-partition my_heytap_a
fastboot delete-logical-partition my_heytap_b
fastboot delete-logical-partition my_manifest_a
fastboot delete-logical-partition my_manifest_b
fastboot delete-logical-partition my_preload_a
fastboot delete-logical-partition my_preload_b
fastboot delete-logical-partition my_product_a
fastboot delete-logical-partition my_product_b
fastboot delete-logical-partition my_region_a
fastboot delete-logical-partition my_region_b
fastboot delete-logical-partition my_stock_a
fastboot delete-logical-partition my_stock_b
fastboot delete-logical-partition odm_a-cow
fastboot delete-logical-partition odm_b-cow
fastboot delete-logical-partition system_a-cow
fastboot delete-logical-partition system_b-cow
fastboot delete-logical-partition system_ext_a-cow
fastboot delete-logical-partition system_ext_b-cow
fastboot delete-logical-partition product_a-cow
fastboot delete-logical-partition product_b-cow
fastboot delete-logical-partition vendor_a-cow
fastboot delete-logical-partition vendor_b-cow
fastboot delete-logical-partition my_carrier_a-cow
fastboot delete-logical-partition my_carrier_b-cow
fastboot delete-logical-partition my_company_a-cow
fastboot delete-logical-partition my_company_b-cow
fastboot delete-logical-partition my_engineering_a-cow
fastboot delete-logical-partition my_engineering_b-cow
fastboot delete-logical-partition my_heytap_a-cow
fastboot delete-logical-partition my_heytap_b-cow
fastboot delete-logical-partition my_manifest_a-cow
fastboot delete-logical-partition my_manifest_b-cow
fastboot delete-logical-partition my_preload_a-cow
fastboot delete-logical-partition my_preload_b-cow
fastboot delete-logical-partition my_product_a-cow
fastboot delete-logical-partition my_product_b-cow
fastboot delete-logical-partition my_region_a-cow
fastboot delete-logical-partition my_region_b-cow
fastboot delete-logical-partition my_stock_a-cow
fastboot delete-logical-partition my_stock_b-cow

fastboot create-logical-partition odm_a 1
fastboot create-logical-partition odm_b 1
fastboot create-logical-partition system_a 1
fastboot create-logical-partition system_b 1
fastboot create-logical-partition system_ext_a 1
fastboot create-logical-partition system_ext_b 1
fastboot create-logical-partition product_a 1
fastboot create-logical-partition product_b 1
fastboot create-logical-partition vendor_a 1
fastboot create-logical-partition vendor_b 1
fastboot create-logical-partition my_carrier_a 1
fastboot create-logical-partition my_carrier_b 1
fastboot create-logical-partition my_company_a 1
fastboot create-logical-partition my_company_b 1
fastboot create-logical-partition my_engineering_a 1
fastboot create-logical-partition my_engineering_b 1
fastboot create-logical-partition my_heytap_a 1
fastboot create-logical-partition my_heytap_b 1
fastboot create-logical-partition my_manifest_a 1
fastboot create-logical-partition my_manifest_b 1
fastboot create-logical-partition my_preload_a 1
fastboot create-logical-partition my_preload_b 1
fastboot create-logical-partition my_product_a 1
fastboot create-logical-partition my_product_b 1
fastboot create-logical-partition my_region_a 1
fastboot create-logical-partition my_region_b 1
fastboot create-logical-partition my_stock_a 1
fastboot create-logical-partition my_stock_b 1

ECHO ###############################
ECHO # Flashing logical partitions #
ECHO ###############################
fastboot flash vendor vendor.img
fastboot flash odm odm.img
fastboot flash system system.img
fastboot flash system_ext system_ext.img
fastboot flash product product.img
fastboot flash my_carrier my_carrier.img
fastboot flash my_company my_company.img
fastboot flash my_engineering my_engineering.img
fastboot flash my_heytap my_heytap.img
fastboot flash my_manifest my_manifest.img
fastboot flash my_preload my_preload.img
fastboot flash my_product my_product.img
fastboot flash my_region my_region.img
fastboot flash my_stock my_stock.img

ECHO #######################
ECHO # Rebooting to system #
ECHO #######################
fastboot reboot
pause