syscfg.txt is intended to contain system-made configuration changes and user configuration changes should be placed in usercfg.txt. Consequently I edited, in the boot partition of the RPi4's SD card mounted in a laptop, usercfg.txt and inserted enable_uart=0 there. Xubuntu 20.04 LTS now boots correctly with Fan Shim mounted.
syscfg.txt is intended to contain system-made configuration changes and user configuration changes should be placed in usercfg.txt. Consequently I edited, in the boot partition of the RPi4's SD card mounted in a laptop, usercfg.txt and inserted enable_uart=0 there. Xubuntu 20.04 LTS now boots correctly with Fan Shim mounted.