Install Magisk on the OnePlus 8T (or any A/B slot device)

main.sh

adb reboot fastboot
sudo env "PATH=$PATH" fastboot getvar current-slot # Returned a in my case
sudo env "PATH=$PATH" fastboot reboot
adb root
adb shell dd if=/dev/block/bootdevice/by-name/boot_a of=/sdcard/Download/boot.img # or boot_b if it was returned above
# Open the Magisk app, click install, select the file from Downloads/boot.img, then click lets go and note the output file path
adb pull /sdcard/Download/magisk_patched-23000_kZmUG.img
adb reboot fastboot
sudo env "PATH=$PATH" fastboot set_active a # Or b if it was returned above
sudo env "PATH=$PATH" fastboot flash boot magisk_patched-23000_kZmUG.img
sudo env "PATH=$PATH" fastboot reboot
rm magisk_patched-23000_kZmUG.img